At what speed is a particle's kinetic energy twice its rest energy?

The total energy of a particle is its rest energy plus the kinetic energy. Its formula is: Et = m^2/sqrt ((1-v^2) / c^2))

The rest energy is equal to the product of mass and the square of light velocity: Er=mc^2.

When the kinetics energy is twice its rest energy this holds:

Et=Er

m^2/sqrt ((1-v^2) / c^2)) = mc^2.

sqrt ((1-v^2) / c^2)) = m/c^2

=> v=sqrt (3/2) c