Explanation:
The slope of any vertical line is always undefined. This is because [tex]\text{slope} = \frac{\text{rise}}{\text{run}}[/tex], and the "run" portion is 0. The run represents the change in x, but we don't have a change in x for vertical lines. The x value is always some constant.
So this is why the run is 0, and it leads to a division by zero error, causing an undefined slope.
