1before firing both the gun car and the shell are in state of rest .
after firing velocity of shell= (200+0) m/s (since velocity of car before firing was zero)
conserving momentum of the shell fired and guncar
0= 49mv+m200
v= -200/49 m/s
now after firing the second shot velocity of the shell = -20049 + 200
conversing momentum of the system (second shell and car)
49m* -20049 = 48m v + m*(-20049 + 200)
solving
v= -200(149+148)