#### Answer

$x=4$

#### Work Step by Step

RECALL:
(1) A vertical line has an undefined slope and has an equation of the form $x=h$ where $h$ is the x-coordinate of all the points on the line.
(2) Vertical lines are parallel to each other.
The line is parallel to the y-axis.
Since the y-axis is a vertical line, its slope is undefined..
This means that the slope of the line we are looking for is also undefined.
The vertical line we are looking for the equation of passes through $(4, 5)$.
The x-coordinate of this point is $4$.
Therefore, the equation of the line we are looking for is $x=4$.