Momentum in special relativity problem

[sapz]

Homework Statement

Hi there.

I added a question as an image, what is the right way to find the momentum?

Homework Equations

The Attempt at a Solution

 

[Simon Bridge]

what is the right way to find the momentum?

You use the energy-momentum relationship.

$E^2 = m^2c^4 + p^2c^2$

The kinetc energy is $K=(\gamma -1)mc^2$ and $E=\gamma mc^2$

 

[sapz]

Im aware of that. However could you please address the question in the picture? Which way is wrong, and why?

 

[Simon Bridge]

I was[/] addressing the pic.
The pic includes the question "What is the momentum of the moving particle?"
You wanted to know the right way to go about it - I told you.

It seems I misunderstood.
Define "right way". (What makes you think there is a wrong way in there?)
The pic shows you how to solve for two contexts in relation to the center-of-mass.