(Solved): At time t=0 a grinding wheel has an angular velocity of 21.0rad/s. It has a constant angular accel ...

At time $t=0$ a grinding wheel has an angular velocity of $21.0\mathrm{rad}/\mathrm{s}$. It has a constant angular acceleration of $27.0\mathrm{rad}/{\mathrm{s}}^2$ until a circuit breaker trips at time $t=1.90\mathrm{s}$. From then on, the wheel turns through an angle of $432\mathrm{rad}$ as it coasts to a stop at constant angular deceleration. What was the wheel's angular acceleration as it slowed down? Express your answer in radians per second per second.