$${K = W = -TR (\theta_f - \theta_i) = -TR ( \frac{1}{2} \alpha t^2) = - \frac{1}{2} TR \alpha t^2 \\ = - \frac{1}{2} (6,0\, \mathrm{N})(0,20\, \mathrm{m})(-24\, \mathrm{rad/s^2})(2,5\, \mathrm{s})^2 \\ = \underline{\underline{90\, \mathrm{J}}}}$$