Coulomb’s law

Two small identical metal spheres carry charges of +$3.0\times10^{-9}\,\text{C}$ and +$5.0\times10^{-9}\,\text{C}$. They are released from rest with their centres $0.020\,\text{m}$ apart in vacuum. Treat one sphere as fixed. Using Coulomb’s law and energy conservation, what is the distance of closest approach of the moving sphere? (Take $k = 9.0\times10^9\,\text{N m}^2\text{C}^{-2}$, mass of moving sphere $= 2.0\times10^{-3}\,\text{kg}$.)