Answer:
5.3 cm
Step-by-step explanation:
Let the charges on the spheres be q and Q.
r = 9.20 cm
F = k Q q / (9.20)^2 ..... (1)
Let the new distance be r' and force F'
F' = 3 F
F' = k Q q / r'^2
3 F = k Q q / r'^2 ...... (2)
Divide equation (1) by (2)
F / 3 F = r'^2 / 84.64
1 / 3 = r'^2 / 84.64
r' = 5.3 cm