Answer
$3x+y=0$
Work Step by Step
Using $y-y_1=m(x-x_1)$ or the Point-Slope Form of linear equations, the equation of the line with the given conditions,
\begin{align*}\require{cancel}
y_1&=
0
,\\x_1&=
0
,\\m&=
-3
,\end{align*}
is
\begin{align*}\require{cancel}
y-0=-3(x-0)
.\end{align*}
In the form $Ax+By=C,$ the equation above is equivalent to
\begin{align*}
y&=-3(x)
\\
y&=-3x
\\
3x+y&=0
.\end{align*}
