What is the rest mass of the composite particle after a relativistic collision?

[Void123]

Homework Statement

A stationary particle of rest mass $$2m_{0}$$ is hit by a particle of rest mass $$m_{0}$$ and kinetic energy $$2m_{0} c^{2}$$. I must find the rest mass of the composite particle afterward.

Homework Equations

Conservation of momentum and energy.

The Attempt at a Solution

I worked problems with identical mass and velocity, but I don't exactly know what to do with this particular problem. I'm still having trouble understanding the explanation my book provided, so I was hoping someone can give me a more lucid elaboration of this whole phenomenon.

 

[Void123]

This is what I have so far.

$$m_{0} \gamma (u) u = M_{0} q \gamma(q)$$

$$m_{0} \gamma (u) + 2m_{0} = M_{0} \gamma (q)$$

$$M_{0}$$ is the rest mass of the composite particle, $$q$$ its velocity, and $$u$$ the velocity of the inertial particle before it strikes the stationary one.

The problem is the ratio $$\frac{M_{0}}{m_{0}}$$, which is what I need, is eliminated.

I don't know where to proceed from here.