A solid body rotates about a stationary axis according to the law = at - bt3, where

Question:

A solid body rotates about a stationary axis according to the law φ = at - bt3, where α = 6.0 rad/s and b = 2.0 rad/s3. Find:
(a) The mean values of the angular velocity and angular acceleration averaged over the time interval between t = 0 and the complete stop;
(b) The angular acceleration at the moment when the body stops