Chapter 3 - Introduction to Graphing - 3.5 Slope - 3.5 Exercise Set - Page 200: 57

$m=\text{undefined}$

Linear equations of the form $x=a$ where $a$ is a real number have a slope, $m$, that is undefined. Hence, the slope of the given equation, $ x=-8 $, is \begin{array}{l} m=\text{undefined} .\end{array}