t K For the function below, find a) the critical numbers, b) the open intervals where the function is increasing; and c) the open intervals where it is decreasing. f(x) 4x-3x-36x+9 a) Find the critical number(s). Select the correct choice below and, if necessary, fill in the answer box to complete your choice. A. The critical number(s) is/are (Type an integer or a simplified fraction. Use a comma to separate answers as needed.) OB. There are no critical numbers.