#### Answer

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#### Work Step by Step

RECALL:
$\text{slope} = \frac{\text{change in y}}{\text{change in x}}$
The points on a vertical line share the same x-value.
This means that for any two given points, the change in x-value is 0.
Since the denominator of $\frac{\text{change in y}}{\text{change in x}}$ is 0, for whatever value of the numerator, the slope will always be undefined (division of zero is not allowed).
Thus, the slope of a vertical line is undefined.