Graphing a first-order model. Consider the first-order equation
y = 1 + 2x1 + x2 - 3x3
(a) Graph the relationship between y and x1 for x2 = 1 and x3 = 3.
(b) Repeat part a for x2 = -1 and x3 = 1.
(c) If a linear model is first-order in three independent variables, what type of geometric relationship will you obtain when E(y) is graphed as a function of one of the independent variables for various values of the other independent variables?