A bullet (mass 3.5 g), traveling horizontally at unknown speed, strikes a ballistic pendulum (that is, a wood block of mass 420 g suspended vertically and originally at rest), see figure below. The bullet quickly comes to rest in the block, and the block+bullet swing to a height of 12 cm above the original position of the block.
(a) What is the speed of block+bullet immediately after the collision?
(b) What was the original speed of the bullet?