angular acceleration problem

tigerlili

1. The problem statement, all variables and given/known data

A flywheel turns through 22 rev as it slows from an angular speed of 6.5 rad/s to a stop. (a) Assuming a constant angular acceleration, find the time for it to come to rest. (b) What is its angular acceleration? (c) How much time is required for it to complete the first 11 of the 22 revolutions?

2. Relevant equations

n revolutions = 2npi radians

2npi= 1/2 alpha(t^2)
omega final^2 = omega initial^2 + 2alpha (2npi)

3. The attempt at a solution

i already got parts a and b, but i am confused about part c
the problem states that the angular acceleration is constant, so why can't i just use the kinematic analogue that i used to find the time in part a to find part c. please help!