Answer
$-170\frac{rad}{s^2}$.
Work Step by Step
Find the initial angular velocity.
$$6500 \frac{rev}{60 s}\frac{2 \pi rad}{rev}=681 \frac{rad}{s}$$
Use the definition of angular acceleration.
$$\alpha=\frac{\Delta \omega}{\Delta t}=\frac{0-681 rad/s}{4.0s}=-170\frac{rad}{s^2}$$
