Answer:
-4 / 0
Explanation:
Given the following question:
(-5, 8), (-5, 4)
In order to find the answer to this question we will use rise over run, substitute the values, and then solve.

(-5, 8), (x1, y1)
(-5, 4), (x2, y2)
4 - 8 = -4
-5 - -5 = 0
m = -4 / 0
Hope this helps.