A plank of mass M is placed over smooth triangular wedge with angle of inclination 30° . block of mass m is connected to the plank by a massless string . a cylinder of mass M and radius R is now placed over plank, such that one end of the string , wrapped over cylinder is fixed and parallel to incline . friction is sufficient enough for pure rolling of cylinder. If system is released from rest with condition that M=4m , find the angular acceleration of cylinder .