You have been asked to design a "ballistic spring system" to measure the speed of bullets. A spring whose spring constant is k is suspended from the ceiling. A block of mass M hangs from the spring. A bullet of mass m is fired vertically upward into the bottom of the block. The spring's maximum compression d is measured from its initial suspended position.

Find an expression for the bullet's speed vB

What was the speed of a $10$ g bullet if the block's mass is $2\mathrm{.}2$ kg and if the spring, with k $=54$ N$/$m$,$ was compressed by $42$ cm$?$