Answer
(a) $(-3, 3)$
(b) $(-\infty, -3) \cup (3, +\infty)$.
(c) The graph has no interval where its value is constant.
Work Step by Step
(a) The graph is rising from the left up to $x=-3$ as well as from $x=3$ to the right. Thus, the function is increasing in the interval $(-3, 3)$.
(b) The graph is falling from the left up to $x=-3$ as well as from $x=3$ to the right. Thus, the function is decreasing in the interval $(-\infty, -3) \cup (3, +\infty)$.
(c) The graph has no interval where its value is constant.