What is the slope of x equals -7?

Answer 1

The slope is undefined.

X = 7 (or any other number) gives you a vertical line. Since slope is defined as the change in y divided by the change in x, or rise over run, for a vertical line you end up dividing by 0, because there is no change in x. Thus, it is undefined.

Answer 2

This is undefined (infinite or debatably complex infinite), since all values line up on the x axis vertically, meaning a division by 0 (since x values remain on the bottom).