Let the total mass of the particle is M and its initial velocity is U. After the disintegration the velocity of the mass of the rocket motor is m (say) and let the mass of the command module be (M-m). Let the final velocity of the command module be V and the velocity of the rocket motor be v.
Then according to the law of conservation of linear momentum
MU = (M-m)V + mv
Also we are given that M= 5, m=4 , (M-m)=1 , U= 4300km/h, (V-v)= 82km/h. then the equation becomes
(5×4300) = V + 4(V-82)
then V= 4365.6