Answer
$y=3$
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 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 $(1, 3)$.
The y-coordinate of the point $(1, 3)$ is $3$.
Thus, equation of the line is $y=3$.