Calculate the average power dissipation in Joules
In part (b) you solved for the instantaneous power \$p(t)\$.

You can calculate the average power as

$$\dfrac{1}{T}\int_t^{t+T}p(t)\mathrm{d}t$$

Where taking T equal to one cycle of the periodic function will give the average over all time. If you are clever you can work out from symmetry how to get the answer using just the period from t = 0 to t = 1 min.

The units of power are watts, not joules, so the answer given in your text is incorrect.