How to Find the Speed of Particles from a Decay Event?

[matrix0978]

Homework Statement

A particle of mass M = 7m decays into two particles of mass m - one travels in the '+y' direction, the other in the -y' direction.
In the rest frame of M, find the total energy(in terms of m) and the speed of each m.

Homework Equations

E = mc²

\frac{u}{c} = \frac{pc}{E}

The Attempt at a Solution

I figured out the mass of each of the small particles is half of the rest energy of the parent particle and it came out to be (3.15x10^17 * m)Joules. I just don't know how to manipulate the equations to find the speed of each m...?

 

[betel]

Welcome to the forum.
Your results will be more easier to understand if you don't insert values for c. Just keep c^2. standing there. There is no need for actual numbers as your are not given m anyway.
Your first relation can be written in a more explicit way relating total energy, rest mass and momentum.
Once you have the momentum you then can relate it to the velocity using your second.