INITIAL K.E=1/2*mV2 (haif *mass*square od velocity)

using conservation of momentum

final momentum=initial momentum........(1)

let final velocity of system(block+bullet)=W

initial momentum=mV

final momentum=(M+m)W

using 1

W=mV/M+m

final KE=1/2*(M+m)W2

change in KE=final -initial

after usual calculation answer will come