Relative particle decay(velocity)

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 dont know how to manipulate the equations to find the speed of each m...?