Answer
$\color{blue}{x=3}$
Work Step by Step
RECALL:
A line with an undefined slope has an equation of the form $x=h$ where $h$ is the x-intercept. All points on the line have $x=h$.
The given line has an undefined slope, so it is a vertical line.
The line contains the point $(3, 8)$.
This means that all points on the line have $x=3$, and that the equation of the line itself is $x=3$.
Thus, the equation of the line is:
$\color{blue}{x=3}$