Answer: 0
Step-by-step explanation:
Slope of x = -3 is infinite / undefined (there is infinite/no defined change in y for a given change in x). Remember, line x = -3 slopes straight up.
Perpendicular means at a 'right angle to'.
The only kind of line that is perpendicular to a vertical line is a horizontal line, which is like y = 15 or y = 5 or something. The slope of such lines is always zero, because there is no change in line's rise for a given run.