CE Physics: conservation of energy cannot be applied to the question of inelastic collision

Two pendula of the same length are arranged as bob A of mass M at the equilibrium position and bob B of mass 2M hang aside of vertical height 0.45m . Bob B is relased and hit the other boba at the equilibrium position. Finally the two ball stick together. Find the common velocity of the bobs just after collision.

If you use the formula: potential energy of B at original position=Kinetic energy of B at the equilibrum= kinetic energy after collision, then you are wrong. Kinetic Energy is not conserve in the system of inelastic collision. So better find the velocity of B just before collision and use conservation of momentum to reach the answer.