Chapter 3 - Introduction to Graphing - 3.3 Graphing and Intercepts - 3.3 Exercise Set - Page 180: 79

$x=4$

RECALL: A vertical line's equation is of the form $x=h$. Its x-intercept is $(h, 0)$, and each point on the line has an x-coordinate of $h$. The given graph is vertical, and every point on the line has an x-coordinate of $4$. This means the equation of the line is $x=4$.