rotation

before collision <--- 7 and w is anticlockwise 7/R
after just after collision --->7 and w is w is anticlockwise 7/R

Now after some time when pure rolling starts let velocity be v.
-(2MR²/5)7+m7R=7MRv/5
v=3

have applied conservation of angular momentum about bottom most point... and net torque about that point is zero.. as friction and gravity passes through that point..