Physics help! The masses m1 and m2 are moving at velocities v1 and v2 along the x-axis. The K' coordinates are also moving along the x-axis. With what equation does the velocity V of the K' coordinates minimize the sum of the kinetic energies of m1 and m2?

I dont get it, it cant be as simple as V=0.5*m1v1+0.5*m2v2+K