A disk rotates about its central axis starting from rest and accelerates with constant angular acceleration. At one time it is rotating at 7 rev/s. 55 revolutions later, its angular speed is 21 rev/s.

Question

A disk rotates about its central axis starting from rest and accelerates with constant angular acceleration. At one time it is rotating at 7 rev/s. 55 revolutions later, its angular speed is 21 rev/s. 
(a) Calculate the angular acceleration. 
(b) Calculate the time required to complete the 55 revolutions mentioned. 
(c) Calculate the time required to attain the 7 rev/s angular speed. 
(d) Calculate the number of revolutions from rest until the time the disk attained the 7 rev/s angular speed.

This is pretty straighforward. What is about it that you do not understand?

what equations do I use? thanks for your help.