Calculate the velocity of the rifle's recoil after firing m=4kg v=? m=0.010kg v=300 m/s

By the principle of conservation of mechanical energy, you must equal the initial energy with the final energy. The initial energy will be before firing with the following data: m = 0.010kg v = 300 m / s. The final energy will be with the mass m = 4kg and the speed to be calculated which will be V = 15 m / s. The answer is attached with the procedure.