Using momentum conservation,\\\\(\\displaystyle m_{car} \\displaystyle u _{car} + \\displaystyle m_{ truck} \\displaystyle u_{ truck}= \\displaystyle m_{ car} \\displaystyle \\displaystyle v _{car} + \\displaystyle m_{truck} \\displaystyle v_{truck}\\\\)\\\\(400 \ imes 72 + 4000 \ imes 9 = -18 \ imes 400 + 4000 \ imes v\\\\)\\\\(v = 18 \\ kmph\\\\)