Since there are no net externalforces acting on the system, we can apply Conservation of Momentum to this problem. In addition, since the collision is given to be "elastic", we can also apply Conservation of Energy: {Conservation Of Momentum}: ----> {Initial System Momentum} = {Final System Momentum} ----> (m_1)*(v_1_init) +(m_2)*(v_2_init) = (m_1)*(v_1_final) +(m_2)*(v_2_final) ----> (2.0)*(16) + (M)*(0) = (2.0)*(-12.0) + (M)*(v_2_final) ----> (M)*(v_2_final) = 56.0 {Initial System Energy} = {Final System Energy} ----> (1/2)*(m_1)*(v_1_init)^2 +(1/2)*(m_2)*(v_2_init)^2 = = (1/2)*(m_1)*(v_1_final)^2 + (1/2)*(m_2)*(v_2_final)^2 ----> (1/2)*(2.0)*(16)^2 +(1/2)*(M)*(0)^2 = = (1/2)*(2.0)*(-12.0)^2 + (1/2)*(M)*(v_2_final)^2 ----> (M)*(v_2_final)^2 = 224.0