were did this come from this is a momentum problem

Question

After some messey algebra it can be said that

V_B = (2 m_A V_o_A)/(m_A + m_B)

V_A = ( (M_A - M_B ) V_o_A )/(M_A + M_B)

were did this come from???

were _B indicates a quantity realtive to B
and _A indicates a quantity realitve to A

and V_o is the intial velocity