Electrostatics: JEE Main 2021 Physics question

Two identical conducting spheres with negligible volume have 2.1 nC and $-$0.1 nC charges, respectively. They are brought into contact and then separated by a distance of 0.5 m. The electrostatic force acting between the spheres is __________ $\times$ 10^$-$9 N. [Given : $4\pi {\varepsilon _0} = {1 \over {9 \times {{10}^9}}}$ SI unit]