Finding the solar radius from temperature and luminosity

This question has eluded me for days; I can't seem to solve it.

A certain star has a temperature twice that of the Sun and a luminosity 55 times greater than the solar value. What is its radius, in solar units?

There is an approximate relation between luminosity, radius and temperature for stars: $$L = 4\pi R^2\sigma_B T_S^4$$