Assuming the light takes essentially no time to reach you, the distance at which the lightning occurred can be calculated by multiplying the speed of sound by the time it takes to hear the thunder:
Impulse=force x time force=mass x acceleration due to gravity force= impulse =3000 x 2.5= ( sorry i don't have a calculator right now so you must calculate this yourself) I converted from kg to g because it is the standard. Hope this helps you.