A car motor is slowing down from 4200 to 1100 rpm over 4 sec. The negative acceleration is approximately 81(I don't have the problem in front of me). How many revolutions does the motor make over those 4 sec. I can't figure this out because the revolutions are getting slower and slower and therefore each revolution takes more and more time.
The units of your "81" number are apparently radians/s^2. That should have been included.
You can use the fact that the average rpm during deceleration is 5300/2 = 2650 rev/min = 277.5 rad/s. Since 4 s = 1/15 minute, the number of engine revolutions made would be
2650/15 = 176.7