The answer is the product of their slope is -1.
First, let's break this down. Perpendicular lines are line with opposite reciprocal slopes. Opposite meaning the inverse sign and reciprocal meaning flipped.
For instance,
If a line had the slope of 5, its inverse would have a slope of -1/5.
Now, if we multiply these numbers together,
5 * -1/5 = -1
The 5's cancel out.
This works for any two perpendicular lines
