We have two points, (9,11) and (9,15).
We have to find the slope of the line that passes through them.
In this case, the line that passes through them is a vertical line.
As the slope m is defined as:
as x1 = x2 in this case (both points have the same x-coordinate), the denominator will be 0 and then the slope will be undefined.
Vertical lines have undefined slopes.
Answer: the slope is undefined as it is a vertical line.