SIR\MADAM PLEASE EXPLAIN ME ABOUT THIS PROBLEM

The speed V of the pendulum bob after the bullet has passed through is given by
MV = m(v - v/2) = mv/2
V = mv/(2M)
To swing all the way to the top,
(1/2) M V² = M g (2L)
V² = 4 L g = m²v²  /4M² 

v ²=  (16 L g M ²/m ²)
v  = 4 (M/m) sqrt (Lg)