Answer
a) $(-\infty,\infty)$, $x\ne-3$, $x\ne3$
b) $f(x)$ is never decreasing.
Work Step by Step
Step 1: Graphically find the values where $f'(x)$ is equal to $0$, positive & negative.
Step 2: $f(x)$ is increasing where $f'(x)$ is positive.
Step 3: $f(x)$ is decreasing where $f'(x)$ is negative. (This function is never negative)
Note: The answer is in interval notation.