## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$x=4$
The equation of a vertical line is always in the form of: $x=a$, where $a$ is constant. Here, the point given is $(4,-5)$, which is on the line. For this point, $x=4$ and this should be true for the other points on the line. The equation is, therefore: $x=4$