#### Answer

$x=3$

#### Work Step by Step

If the slope is undefined, we can surely state that the line is vertical.
The equation of a vertical line is always in the form of:
$x=a$, where $a$ is constant.
Here, the point given is $(3,8)$, which is on the line.
For this point, $x=3$ and this should be true for the other points on the line.
The equation is, therefore:
$x=3$