The electrostatic force between two charges q1 and q2 is given by
[tex]F=k_e \frac{q_1 q_2}{r^2} [/tex]
where [tex]k_e =8.99 \cdot 10^9 N m^2 C^{-2} [/tex] is the Coulomb's constant and r is the distance between the two charges.
If we use F=19.2 N and q1=q2=-3.0 C, we can find the value of r, the distance between the two charges by re-arranging the previous formula:
[tex]r= \sqrt{k_e \frac{q_1 q_2}{F} }= \sqrt{ 8.99 \cdot 10^9 N m^2 C^{-2} \frac{(-3.0C)^2}{19.2 N} } =6.49 \cdot 10^4 m=64.9 km [/tex]
