I am looking for the correct formula for this question. An 8.0 g bullet at 400 m/s passes through a heavy block of wood in 4.0 x (10 to negative 4) s...

I am looking for the correct formula for this question.  An 8.0 g bullet at 400 m/s passes through a heavy block of wood in 4.0 x (10 to negative 4) s and emerges with a speed of 100 m/s. (a) With what average force did the wood oppose the motion of the bullet? (b) How thick is the block of wood?