Answer
$x=4$, or, in general form, $x-4=0$
Work Step by Step
Through $(4,5);$ parallel to the $y$-axis
.
A point through which the line passes and the fact that the line is parallel to the $y$-axis are known.
If the line is parallel to the $y$-axis, then it's completely vertical and its equation can be represented just by making $x$ equal to the $x$ coordinate of some point through which the line passes.
The equation of this line is:
$x=4$, or, in general form, $x-4=0$
