a
(a)\({v_1} = \left( {\frac{{{m_1} - {m_2}}}{{{m_1} + {m_2}}}} \right)\,{u_1} + \left( {\frac{{2{m_2}}}{{{m_1} + {m_2}}}} \right)\,{u_2}\) and
\({v_2} = \left( {\frac{{2{m_1}}}{{{m_1} + {m_2}}}} \right)\,{u_1} + \left( {\frac{{{m_1} - {m_2}}}{{{m_1} + {m_2}}}} \right)\,{u_2}\)
on putting the values \({v_1} = 6\,m/s\) and \({v_2} = 12\,m/s\)