If the shaft is subjected to a torque of $M=$ $\left(15 t^{2}\right) \mathrm{N} \cdot \mathrm{m}$, where $t$ is in seconds, determine the angular velocity of the assembly when $t=3 \mathrm{~s}$, starting from rest. Rods $A B$ and $B C$ each have a mass of $9 \mathrm{~kg}$.