Chapter 5 - Linear Functions - 5-6 Parallel and Perpendicular Lines - Practice and Problem-Solving Exercises - Page 332: 33

blue line slope: 0 red line slope: undefined $x= 3$

(0,5) and (4,5) $m = (5-5)/(4-0)$ $m = 0/4 = 0$ The blue line has a slope of 0, so the new line is perpendicular to the blue line. The new line has an undefined slope as the new line is completely vertical.