Why am I getting a cannot be resolved error on my code?

package application;

import javafx.application.Application;

import javafx.geometry.Insets;

import javafx.scene.Scene;

import javafx.scene.control.$*$;

import javafx.scene.layout.GridPane;

import javafx.stage.Stage;

public class RoadTripCostEstimator extends Application $\{$

public static void main$($String$[]$ args$)\{$

Application.launch$($args$)$;

$\}$

@Override

public void start$($Stage primaryStage$)\{$

primaryStage.setTitle$("$Road Trip Cost Estimator"$)$;

GridPane grid $=$ new GridPane$()$;

grid.setHgap$(10)$;

grid.setVgap$(10)$;

grid.setPadding$($new Insets$(10,10,10,10))$;

Label distanceLabel $=$ new Label$("$Distance:$")$;

TextField distanceField $=$ new TextField$()$;

ComboBox distanceUnitCombo $=$ new ComboBox;

distanceUnitCombo.getItems$().$addAll$("$Miles$",$ "Kilometers"$)$;

Label gasolineCostLabel $=$ new Label$("$Gasoline Cost:"$)$;

TextField gasolineCostField $=$ new TextField$()$;

ComboBox gasolineCostUnitCombo $=$ new ComboBox;

gasolineCostUnitCombo.getItems$().$addAll$("$Dollars$/$Gal$",$ "Dollars$/$Liter$")$;

Label gasMileageLabel $=$ new Label$("$Gas Mileage:"$)$;

TextField gasMileageField $=$ new TextField$()$;

ComboBox gasMileageUnitCombo $=$ new ComboBox;

gasMileageUnitCombo.getItems$().$addAll$("$Miles$/$Gal$","$Km$/$Liter$")$;

Label numberOfDaysLabel $=$ new Label$("$Number of Days:"$)$;

TextField numberOfDaysField $=$ new TextField$()$;

Label hotelCostLabel $=$ new Label$("$Hotel Cost per Day:"$)$;

TextField hotelCostField $=$ new TextField$()$;

Label foodCostLabel $=$ new Label$("$Food Cost per Day:"$)$;

TextField foodCostField $=$ new TextField$()$;

Label attractionsCostLabel $=$ new Label$("$Attractions Cost:"$)$;

TextField attractionsCostField $=$ new TextField$()$;

Button calculateButton $=$ new Button$("$Calculate$")$;

Label resultLabel $=$ new Label$("$Total Trip Cost:"$)$;

TextField resultField $=$ new TextField$()$;

resultField.setEditable$($false$)$;

grid.add$($distanceLabel$,0,0)$;

grid.add$($distanceField$,1,0)$;

grid.add$($distanceUnitCombo$,2,0)$;

grid.add$($gasolineCostLabel$,0,1)$;

grid.add$($gasolineCostField$,1,1)$;

grid.add$($gasolineCostUnitCombo$,2,1)$;

grid.add$($gasMileageLabel$,0,2)$;

grid.add$($gasMileageField$,1,2)$;

grid.add$($gasMileageUnitCombo$,2,2)$;

grid.add$($numberOfDaysLabel$,0,3)$;

grid.add$($numberOfDaysField$,1,3)$;

grid.add$($hotelCostLabel$,0,4)$;

grid.add$($hotelCostField$,1,4)$;

grid.add$($foodCostLabel$,0,5)$;

grid.add$($foodCostField$,1,5)$;

grid.add$($attractionsCostLabel$,0,6)$;

grid.add$($attractionsCostField$,1,6)$;

grid.add$($calculateButton$,0,7)$;

grid.add$($resultLabel$,0,8)$;

grid.add$($resultField$,1,8)$;

calculateButton.setOnAction$($e $->\{$

double distance $=$ Double.parseDouble$($distanceField$.$getText$())$;

double gasolineCost $=$ Double.parseDouble$($gasolineCostField$.$getText$())$;

double gasMileage $=$ Double.parseDouble$($gasMileageField$.$getText$())$;

int numberOfDays $=$ Integer.parseInt$($numberOfDaysField$.$getText$())$;

double hotelCost $=$ Double.parseDouble$($hotelCostField$.$getText$())$;

double foodCost $=$ Double.parseDouble$($foodCostField$.$getText$())$;

double attractionsCost $=$ Double.parseDouble$($attractionsCostField$.$getText$())$;

double totalTripCost $=$ calculateTotalTripCost$($distance$,$ gasolineCost, gasMileage, numberOfDays, hotelCost, foodCost, attractionsCost$)$;

resultField.setText$($String$.$format$("\%\mathrm{.}2$f$",$ totalTripCost$))$;

$\})$;

Scene scene $=$ new Scene$($grid$,400,400)$;

primaryStage.setScene$($scene$)$;

primaryStage.show$()$;

$\}$

private double calculateTotalTripCost$($double distance, double gasolineCost, double gasMileage, int numberOfDays, double hotelCost, double foodCost, double attractionsCost$)\{$

double gasolineCostInLiters $=($distance $*($distanceUnitCombo$.$getValue$().$equals$("$Miles$")?1\mathrm{.}60934$ : $1))/($gasMileage $*($gasMileageUnitCombo$.$getValue$().$equals$("$Miles per Gallon"$)?1$ : $0\mathrm{.}264172))$;

double totalTripCost $=($gasolineCostInLiters $*($gasolineCostUnitCombo$.$getValue$().$equals$("$Dollars per Gallon"$)?3\mathrm{.}78541$ : $0\mathrm{.}264172)*$ gasolineCost$)$

$+($hotelCost $+$ foodCost$)*$ numberOfDays $+$ attractionsCost;

return totalTripCost;

$\}$

$\}($hotelCost $+$ foodCost$)*$ numberofDays $+$ attractionsCost;

return totaltripCost;

$\}$

Exception in thread "JavaFx Application Thread" java.lang.Error: Unresolved compilation problems:

distanceUnitCombo cannot be resolved

gasMileageUnitCombo cannot be resolved

gasolineCostunitCombo cannot be resolved

at application.RoadTripCostEstimator.calculateTotalTripCost $($RoadTripCostEstimator$.$java:$101)$

at application.RoadTripCostEstimator.lambda$$$0$