Answer
$\alpha = -66~\frac{2}{3}~rev/min^2$
Work Step by Step
We can find the angular acceleration:
$\omega_f = \omega_0+\alpha~t$
$\alpha = \frac{\omega_f - \omega_0}{t}$
$\alpha = \frac{0 - 33~\frac{1}{3}~rev/min}{0.50~min}$
$\alpha = -66~\frac{2}{3}~rev/min^2$
