#### Answer

$y=5$

#### Work Step by Step

RECALL:
(1) Perpendicular lines have slopes whose product is $-1$ (or negative reciprocal slopes).
(2) A horizontal line has a slope of $0$ and has an equation of the form $y=k$ where $k$ is the y-coordinate of all the points on the line.
(4) Horizontal lines are parallel to each other.
The line is parallel to the x-axis.
Since the axis is a horizontal line, its slope is $0$.
This means that the slope of the line we are looking for is also $0$.
The horizontal line we are looking for the equation of passes through $(4, 5)$.
The y-coordinate of this point is $5$.
Therefore, the equation of the line we are looking for is $y=5$.