Mass of body (m1)=m; Velocity of first body (u1)=3km/ hour; Mass of second body at rest (m2)=2m and velocity of second body(u2)=0. After combination, mass of the body M=m+2m=3m From the law of conservation of momentum, weget Mv=m1u1+m2u2 or 3mv=(m×3)+(2m×0)=3m or v=1km/ hour.