#### Answer

(a) $\alpha = -100~rad/s^2$
(b) $\alpha = 0$
(c) $\alpha = 50~rad/s^2$

#### Work Step by Step

The angular acceleration is equal to the slope of the angular velocity versus time graph.
(a) At t = 1 s:
$\alpha = \frac{\Delta \omega}{\Delta t}$
$\alpha = \frac{-200~rad/s}{2~s}$
$\alpha = -100~rad/s^2$
(b) At t = 3 s:
$\alpha = 0$
(c) At t = 5 s:
$\alpha = \frac{\Delta \omega}{\Delta t}$
$\alpha = \frac{150~rad/s}{3~s}$
$\alpha = 50~rad/s^2$