#### Answer

Undefined

#### Work Step by Step

We are given that $x=4$. To find the slope of a line, we can take two points on the line and calculate the slope using the slope formula.
$m=\frac{y_{2}-y_{1}}{x_{2}-x_{1}}$
We know that two points on this line will be (4,6) and (4,8), since all points on this line will have an x-coordinate of 4, no matter the value of y.
$m=\frac{8-6}{4-4}=\frac{2}{0}$
So the slope of this line is undefined. In general, the slope of a vertical line, which is in the form $x=c$, has an undefined slope.