Finding angular acceleration from deceleration

[Sneakatone]

a-c are correct but I don't know how to find part d.

d)when turned off, it decelerates to a stop in 39s. What is its average angular acceleration during this time?

I treated this part as part c to just find angular acceleration like normal.
371.75 rad/s / 39s=9.51 rad/s^2.
should it be negative because it decelerates?

 

[Ampere]

should it be negative because it decelerates?

Yep!

Angular acceleration is defined as (change in angular speed)/(change in time). The speed goes from 371 rad/s to 0, that gives you a negative number.

 

[Sneakatone]

Oh wow. Thanks!