Here in all such collision type question we can use momentum conservation as we can see that there is no external force on this system
[tex]m_1v_{1i} + m_2v_{2i} = m_1v_{1f} + m_2v_{2f}[/tex]
as we know that
[tex]m_1 = 5 kg[/tex]
[tex]m_2 = 2 kg[/tex]
[tex]v_{1i} = 2.8 m/s[/tex]
[tex]v_{2i} = 0 m/s[/tex]
now from above equation we have
[tex]5(2.8) + 2(0) = m_1v+ m_2v[/tex]
[tex]14 = (5+ 2) v[/tex]
[tex]v = 2 m/s[/tex]
so the speed of combined system is 2 m/s
