Answer
$(-\infty,-3)$
Work Step by Step
The function decreases in 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 $x$ between $-\infty$ and $-3$. Since $-3$ is not included in the interval as it is represented by an open circle, a parenthesis will be used on its side. On the other end, the symbol $-\infty$ is always supported by a parenthesis.
Therefore, the function is decreasing in the interval $(-\infty,-3)$.
