The slope is the change in Y over the change in X.
A point is written as ( X,Y), where the first number is the X and the second number is the Y.
You have:
(5,3) and (5,-9)
The change in Y is -9 - 3 = -12
The change in X is 5-5 = 0
The slope becomes -12/0
Because you have a 0 on the bottom of the fraction the slope is undefined because you cannot divide by 0.
The slope is undefined.