Answer: Undefined
Work Shown
(x1,y1) = (18,-7) and (x2,y2) = (18,2)
m = slope = rise/run = (change in y)/(change in x)
m = (y2 - y1)/(x2 - x1)
m = (2 - (-7))/(18 - 18)
m = (2 + 7)/(18 - 18)
m = 9/0
m = Undefined
The slope is undefined because we cannot divide by zero. The slope of any vertical line is undefined.