Answer
The angular acceleration is $190~rad/s^2$
Work Step by Step
We can express the angular velocity in units of $rad/s$:
$\omega = 7200~rpm \times \frac{2\pi~rad}{1~rev} \times \frac{1~min}{60~s} = 240\pi~rad/s$
We can find the angular acceleration $\alpha$:
$\omega_f = \omega_0+\alpha~t$
$\alpha = \frac{\omega_f - \omega_0}{t}$
$\alpha = \frac{240\pi~rad/s - 0}{4.0~s}$
$\alpha = 190~rad/s^2$
The angular acceleration is $190~rad/s^2$
