Particle Decay/4-Momenta Homework Help

[Eldara]

Im stuck on the first question of one of my homework sheets. Partly due to me being horrific at particle physics and lack of sleep (yay exam period) i just can't figure out what's happening and just been staring at it for an hour.

Sheet: https://docs.google.com/file/d/0B6FGt5Eo0GHUdmpMbnhsVlU1QzA/edit

Starting with question 1.

If you re arrange (2) you can get

wx^2 = -Q^2 +mp^2 + 2mpv.

Using M = E - K

I can then go to

Wx^2 = -Q^2 + mp^2 + 2v(Ep-Kp)

This is where i hit a wall as it wants me to show

Wx^2 = -Q^2 + m^2 + Ep (E-E') -2kp(k=k')

it then asks me to use (2) again but i already sued that trying to get to the above.

TL;DR I am confused ;(

 

[mfb]

You can use the same equation more than once, if you like.
Anyway: You need some expression for v. Can you calculate this energy transfer in terms of the energies and momenta given in the problem? What is the energy before the collision, and afterwards?