Answer
$1; 5$
Work Step by Step
1. $f$ is continuous on the closed interval $[0,8]$.
2. $f$ is differentiable on the open interval $(0,8)$.
3. $f(0) = 3$ and $f(8) = 3$
Therefore $f$ satisfies the three hypotheses of Rolle's Theorem.
Then there is a number $c$ in $(0,8)$ so that $f'c)=0$.
From the graph we find that there are two such values of $c$:
$c_1=1$
$c_2=5$.