Physics problem. I got the first part but not the second?

[KJK]

A person is riding a bicycle, and its wheels have an angular velocity of 15.1 rad/s. Then, the brakes are applied and the bike is brought to a uniform stop. During braking, the angular displacement of each wheel is 11.1 revolutions. (a) How much time does it take for the bike to come to rest? (b) What is the anguar acceleration (in rad/s2) of each wheel?

I got a) which was 9.24 seconds but I can't seem to get the right answer for b. Please explain and help

[Matty]

you use the rotational version of your old equation d=1/2at^2+Vi*t

you know t, the rotational version of d is the angle traveled through, which in this case is 11.1 revolutions, or rather 69.7433 radians, t you found is 9.24 seconds, Vi is actually initial angular velocity 15.1 rad/s, and a will be the angular acceleration you're looking for