considering the magnetic force is providing the centripetal force,
we get, mv^2/r = qvB
so, r = mv/qB
as the Kinetic energy, K of the particle is given by.5mv^2
so, v = sqrt(2K/m)
substituting the in radius equation,
r = m*sqrt(2K/m)/qB
r = sqrt(2Km)/qB