Answer
The slope of a vertical line is undefined because the points on the line have the same x-coordinate, so the denominator of the slope is 0, which is undefined.
Work Step by Step
For example, using the points (x,a) and (x,b), we use the slope formula $m=\frac{y_2-y_1}{x_2-x_1}$ to get $m=\frac{b-a}{x-x}=\frac{b-a}{0}$
