Two identical conducting spheres with negligible volume have 2.1 nC and -0.1 nC charges, respectively. They are brought into contact and then…

Two identical conducting spheres with negligible volume have $\displaystyle 2.1$ nC and -$\displaystyle 0.1$ nC charges, respectively. They are brought into contact and then separated by a distance of $\displaystyle 0.5$ m . The electrostatic force acting between the spheres is \_\_\_\_ $\displaystyle \times 10^{-9} \mathrm{~N}$. [Given : $\displaystyle 4 \pi \varepsilon_0=\frac{1}{9 \times 10^9}$ SI unit]