#### Answer

$y=1$

#### Work Step by Step

RECALL:
A horizontal line has a slope of zero and has an equation of the form $y=k$ where $k$ the y-coordinate of all the points on the line.
The line has a slope of zero therefore the line must be horizontal.
The horizontal line passes through the point $(5, 1)$.
The y-coordinate of the point $(5, 1)$ is $1$.
Thus, the equation of the line is $y=1$.