Question

How do you find the slope perpendicular to `y=-7`?

Answer 1

Any x = line will be perpendicular to y = -7, x=3, x = 0 x =-3 etc...
The y line is horizontal and x lines are vertical and therefore will be perpendicular to each other.

Explanation:

The line y = -7 is a horizontal line running through the y axis at -7.

This line has a slope of 0 because it does not rise.
Any x = line will run vertically on the coordinate grid and therefore be perpendicular to the y = -7 line. These vertical lines have no slope as they have no run.