Answer
$(-\infty,-1]$
Work Step by Step
The function decreases on an interval when, moving from left to right, the y-values continue to get smaller for x-values in that interval. For the graph in the question, the y-values decrease for all values of $x\leq-1$. Since $-1$ is included in the interval, a square bracket will be used on its side while on the other end, the symbol $-\infty$ will be used which is always supported by a parenthesis.
Therefore, the function is increasing on the interval $(-\infty,-1]$.