Answer
$y=-x-3$
Work Step by Step
A $y-$intecept of $(
0,-3
)$ implies that $b=
-3
.$
Using $y=mx+b$ or the slope-intercept form of linear equations where $b$ is the $y-$intercept and $m$ is the slope, the equation of the line with $m=
-1
,$ and $b=
-3
$ is
\begin{array}{l}\require{cancel}
y=-x-3
.\end{array}