Tolong dibantu nomer 50. Makasih

[tex]\displaystyle m_a=2m_b\\v_a=+3\,\text{m/s}\\v_b=-6\,\text{m/s}\\\\v'=?\\\\m_av_a+m_bv_b=(m_a+m_b)v'\\2m_b\cdot3-m_b\cdot6=(2m_b+m_b)v'\\6m_b-6m_b=3m_bv'\\\boxed{\boxed{0\,\text{m/s}=v'}}[/tex]