The momentum of the bullet will be conserved, so we want to find the momentum of the block and bullet system just after collision and set it equal to the momentum of the bullet. First we will find 
, the velocity of the block and bullet just after collision which caused the fall.
We can find the time it takes for the block to fall from the post by observing that the velocity with which the bullet strikes the block doesn't effect the initial velocity in the y-direction (= to zero).
Using t=0.553s we can now find the velocity of the block in the x-direction using the distance from the base of the post the block landed, and use it to find the momentum of the block which will equal the momentum of the bullet just before the collision. Since there is no acceleration in the x-direction:
