The problem statement:
Two protons with kinetic energies $W_{k1}=4GeV$ and $W_{k2}=2GeV$ colide and form new particles. What is the mass of newly born particles? There are as many as possible new particles.
Relevant equations here are: \begin{align} E_{before}&=E_{after}\\ p_{before}&=p_{after}\\ E^2 &= {E_0}^2 + p^2c^2 \longleftarrow \substack{\text{Lorentz invariant}} \end{align}
First I wrote the energy conservation law: \begin{align} E_{before} &= E_{after}\\ E_{k1} + E_{k2} + 2E_{0p} &= \sqrt{{E_{0~after}}^2 +p^2c^2}\longleftarrow\substack{\text{Here the $E_{0~after}$ is a full}\\\text{rest energy after colision}}\\ E_{k1} + E_{k2} + 2E_{0p} &= \sqrt{\smash{\underbrace{\left( 2E_{0p} + E_{0m} \right)^2}_{\substack{\text{after collision we have}\\ \text{2 $p^+$ and new mass $m$}}}} +p^2c^2} \\ \\ \\ \\ \\ \end{align}
At this point I am not sure what to do with the last part $p^2c^2$. The only thing I came up with was to set:
\begin{align} pc &= \sqrt{{E_{k~before}}^2 - 2E_{k~before}E_{0~before}}\\ pc &= \sqrt{\left(E_{k1} + E_{k2}\right)^2 - 2\left(E_{k1}+E_{k2}\right)2E_{0p}}\\ \end{align}
If I continued with this calculation I got $W_{0m}=1.87GeV$ while in the solutions it is said to be $W_{0m}=5.634GeV$. Where did I go wrong?