Electrostatics: JEE Main 2017 Physics question

The energy stored in the electric field produced by a metal sphere is 4.5 J. If the sphere contains 4 $\mu$C charge, its radius will be : [ Take : ${1 \over {4\,\pi { \in _0}}} =$ 9 $\times$ 10⁹ N $-$ m²/C² ]