Answer
$(-\infty,0]$
Work Step by Step
First I graph the function.
Then, according to the graph of the function: $f(x)\leq0$ in the interval $(-\infty,0]$ because the graph of the function is under or on the x-axis ($y=0$) on this interval.
$0$ and $-2$ are included because the inequality involves less than or equal to.