Chapter 5 - Linear Functions - 5-6 Parallel and Perpendicular Lines - Practice and Problem-Solving Exercises - Page 331: 13

Perpendicular

Two non-vertical lines are perpendicular if the product of their slopes is $-1$. The equations $y=x+11$ and $y=-x+2$ have slopes of $1$ and $-1$, respectively. Since $1\times-1=-1$, the lines are perpendicular.