I'm really struggling with a engineering science question about finding angular retardation. The question is -

a rotating system has a mass of 160kg and at all speeds the frictional resistance is equivalent to a torque of o.9Nm.

During a test the speed was increased from 160rpm to 220rpm in 4 seconds by a applied torque of 10.7 Nm.

If the system is rotating at 220 rpm and the driving torque is eliminated find the angular retardation.

I guess it has something to do with the frictional resistance but I don't know how to incorporate it, all other work I have done I've used a time of retardation and the speeds to figure it out but in this question I have no time.

Thanks guys