Consider the first-order model equation in three quantitative independent variables E1y2 = 2 - 3x1 + 5x2 - x3

a. Graph the relationship between y and x3 for x1 = 2 and x2 = 1.

b. Repeat part a for x1 = 1 and x2 = -2.

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 E1y2 is graphed as a function of one of the independent variables for various combinations of the other independent variables?