Answer: 6
Step-by-step explanation:
Slope is found by doing:

(x1 and y1 must be from the same ordered pair and x and y must be from the same ordered pair, however point (x1,y1) and (x,y) can be used interchangeably)
Let us say that (x1,y1) is (3,18)
And (x,y) is (5,30)

The answer would be 9. Because, you put 9 in each row adding up to 18 but it would still be a rectangle.
Hope this helps! :)
Perpendicular hope it helps