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

##x=c##, where ##c## is any constant, implies that the is undefined.

##y=-7## is a constant function. Hence, when you see its graph, you'll see a horizontal line. Then, that means that a perpendicular line must be completely vertical. But such a slope is undefined, since it could mean that it's going up extremely fast, or it's going down extremely fast.