Given,
Total Mass, Mtotal=9kg
Initial velocity, Vinitial=50ms−1
Lighter Mass =M1
Final velocity of (M2=6kg)part, V2=80ms−1
from conservation of momentum
Final momentum = initial momentum
M2V2+M1V1=MtotalVinitial
6×80+3×V1=9×50
V1=−10ms−1
Final velocity of lighter mass is −10ms−1