Answer
$x=4$.
Work Step by Step
If a line is vertical, then its equation must be in the form $x=h$ where $h$ is a real number.
All points on the line have the same $x$-coordinate.
The line is vertical and contains the point $(4,-5)$, the $x$-coordinate of which is $4$.
Hence, the equation of the line must be $x=4$.
