Answer
$(-\infty,4]$
Work Step by Step
The function increases on an interval when, moving from left to right, the y-values continue to get larger for x-values in that interval. For the graph in the question, the y-values increase for all values of $x\leq4$. Since $4$ 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 in the interval $(-\infty,4]$.
