a bus having mass of 3000kg is moving at 30m/sec.find the momentum of the bus.

1)at the starting point from rest.
2)at the final state while motion at 30m/sec.
3)find the difference of moment in above (1) and (2)

You state that the bus is moving at 30 m/s . Then you state that it is "at rest" at a starting point. Are we supposed to assume that it is accelerating between the starting and final points?

Momentum is mass times velocity. Do what you can with that. Your question is too vague.