Answer
$x+y=0$
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{array}{l}\require{cancel}
y_1=3
,\\x_1=-3
,\\m=-1
,\end{array}
is
\begin{array}{l}\require{cancel}
y-3&=-1(x-(-3))
\\
y-3&=-1(x+3)
\end{array}
In the form $Ax+By=C$ or the Standard Form of Linear Equations, the equation above is equivalent to
\begin{align*}
y-3&=-x-3
\\
x+y&=-3+3
\\
x+y&=0
\end{align*}
