Answer
(a) $(-\infty,-3)$
(b) $(-3,\ \infty)$
(c) None
Work Step by Step
(a)
We see from the graph see that the largest interval over which $f$ is decreasing is all $x$ values before -3:
$(-\infty,-3)$
(b)
We see from the graph see that the largest interval over which $f$ is increasing is all $x$ values after -3:
$(-3,\infty)$
(c)
From the graph we see that there is no interval over which $f$ is constant.