$${K = \frac{1}{2} I \omega^2 = \frac{1}{2}(\frac{1}{2} MR^2)(\alpha t)^2 = \frac{1}{4} M (R \alpha)^2 \\ = \frac{1}{4}(2,5\, \mathrm{kg})[(0,20\, \mathrm{m})(-24\, \mathrm{rad/s^2})/2,5\, \mathrm{s})]^2 \\ = \underline{\underline{90\, \mathrm{J}}}}$$