How Do You Calculate Kinetic Energy in Relativistic Collisions?

[1bigman]

Homework Statement

We have a particle mass $m$ with kinetic energy $k$ colliding with a stationary particle of mass $2m$. The collision creates a composite mass of $\sqrt{17}m$. Find the value of $k$

Homework Equations

The Attempt at a Solution

I have tried using mass-energy conservation and also momentum conservation, but I can't seem to get the required result of $k=2mc^2$

 

[vela]

Please show your work. Simply generically describing what you tried unsuccessfully isn't helpful at all.

 

[1bigman]

Please show your work. Simply generically describing what you tried unsuccessfully isn't helpful at all.

My apologies. Fairly new round here... I've figured it out now, but many thanks anyways