Answer
$y=-1$
Work Step by Step
Horizontal lines have a slope of $0$.
Using $y-y_1=m(x-x_1)$ or the Point-Slope Form, then the equation of the line through $(
3,-1
)$ with slope $
0
$ is
\begin{array}{l}
y-(-1)=0(x-3)
\\\\
y+1=0
\\\\
y=-1
.\end{array}
