So to do this, I solved for the total rest energy of both particles.
The rest energy of Kaon + K = Rest energy of Proton
Rest energy of Proton - Rest Energy of Kaon = K
K = P^2/2m
((Rest Energy of Proton - Rest Energy of Kaon)*(2*Mass of Kaon))^1/2 = P
P = mv/(sqrt(1-v^2/c^2)
If I do this and do all the algebra correct and solve for v, will this method give me the correct answer? I got the wrong answer but I suck at numbers X_x.
I realize after looking at the answers on the website this is a poor way to do this problem with GRE time constraints, I just want to know if this thought process is flawed or not.