Chapter 1 - Graphs - 1.3 Lines - 1.3 Assess Your Understanding - Page 31: 58

$x=3$

If the slope is undefined, then (by convention) the equation must be in the form $x=h$ where $h$ is a real number. The slope of the given line is undefined and the line contains $(3,8)$, the $x$-coordinate of which is $3$. Hence, the equation of the line must be $x=3$.