Consider the following first-order model equation in three quantitative independent variables:

E1y2 = 2 + 4x1 - 2x2 - 5x3

a. Graph the relationship between y and x1 for x2 = -2 and x3 = 2.

b. Repeat part a for x2 = 3 and x3 = 3.

c. How do the graphed lines in parts a and b relate to each other? What is the slope of each line?

d. If a linear model is first order in three independent variables, what type of geometric relationship will you obtain when you graph E(y) as a function of one of the independent variables for various combinations of values of the other independent variables?