Answer
$y$-intercept: $y = 0$, slope: $m = \displaystyle -\frac{1}{3}$
Work Step by Step
The slope-intercept form of a line: $y = mx + b$ where $m$ is the slope and $b$ is the $y$-intercept.
Isolate for $y$,
$x+3y=0$
$3y=-x$
$\displaystyle y = -\frac{1}{3}x$
Thus, $y$-intercept: $y = 0$, slope: $m = \displaystyle -\frac{1}{3}$