The Coulomb's force acting between two charges is
[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, q1 and q2 the two charges, and r the distance between them.
Using [tex]q_1 = q_2 = -3 C[/tex], we can find the distance between the two charges when the force is F=19.2 N:
[tex]r= \sqrt{k_e \frac{q_1 q_2}{F} }= \sqrt{ 8.99\cdot 10^9 \frac{(-3.0C)^2}{19.2 N} }=6.5 \cdot 10^4 m = 65 km [/tex]
