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 $\_\_\_\_$ $\times 10^{-9} \mathrm{~N}$. $$ \left[\text { Given : } 4 \pi \varepsilon_0=\frac{1}{9 \times 10^9} \text { SI unit }\right] $$