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

$x = c$, where $c$ is any constant, implies that the slope 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.