Forgive me but they deleted my answer for it did not have any work to be shown providing you know how to do Algebra you get X= 12 as your outcome. <span />
There are two ways to approach this.
Option #1:
Use the slope formula
.

That value is undefined, so the slope is undefined.
Option #2:
Because the two points have the same x-value, they will be vertically above/below one another. This means your line is a vertical line and the slope of a vertical line is undefined.
Either way, your slope is undefined.