Answer
a) $(3,5)$
b) $(-\infty,3)$ & $(5,\infty)$
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.