We are given:
The line passes through the points (9,4) and (9,-5)
We know that:
Slope of a line = (change in y) / (change in x)
Finding the slope:
Change in x = final x - initial x
Change in x = 9 - 9 = 0
Slope of line = (change in y) / 0
because division by 0 is undefined:
Slope of the line is Undefined
We could've solved for the change in y but there's no reason to because no matter what's in the numerator, as long as the denominator is 0, the value is undefined
