Help with determining other mass of a relativistic collision

[dgresch]

Homework Statement

A 3.000 u (1 u = 931.5 MeV/c2) object moving to the right through a laboratory at
0.8c collides with a 4.000 u object moving to the left through the laboratory at
0.6c. Afterward there are two objects, one of which is a 6.000 u object at rest.
Determine the mass of the other object

Homework Equations

CONSERVATION OF ENERGY
\gammam₁c²+\gammam₂c²=\gammam₃c²+\gammam₄c²

The Attempt at a Solution

Each gamma is different because of different speeds but for the purpose of time management and the fact that past this I don't know what to do, I left them as gamma.
\gamma2794.5Mev+\gamma3726MeV=5589Mev+\gammam₄c²

Am I supposed to assume both objects are at rest? Doesn't seem like that would be the case because the other object would just be 1u. Any thoughts anyone?

 

[fzero]

You will also need to apply conservation of momentum. Together with conservation of energy, you will have two equations for the two unknowns \gamma_4, m_4.