Answer
$(-1,3)$
Work Step by Step
We are looking for the values where $f(x)<0$, or where it is negative. The points that this will be focused on are the zeros of the function - where f(x) = 0. We can see that those lie at $x=-1, x=3$
Between those values, the graph lies below 0, and is negative, so we know that x is between -1 and 3. Furthermore, know that $f(x)<0$ means that f(x) wouldn't equal zero, so the points -1 and 3 themselves wouldn't lie in the range.
So, we know that x > -1 and x < 3, so the answer is $(-1,3)$
