Answer
a) $(-\infty,-8)$, $(-6,-2\frac{1}{2})$ & $(-1\frac{1}{2},\infty)$
b) $(-8,-6)$ & $(-2\frac{1}{2},-1\frac{1}{2})$
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.
Note: The answer is in interval notation.