An 8.0 g bullet is fired horizontally into a 9 kg block of wood and sticks in it.The block,which is free to move,has a velocity of 40cm/s after impact.Find the initial velocity of a bullet.

You know the momentum (total mass) of the bullet/block combo after the collision, and you know the momentum of the block before the collision(zero).

Momentumbullet+momentumblock=totalmomentumafter