Answer
To increase force decreases the distance between the charged particles.
Explanation
According to coulomb's law
[tex]F = kq_1 q_2/r^2[/tex]
The force is directly proportional to the potential of two charges and inversely proportional to the distance between the charges. So if we want to increase the force 9 times, we have to reduce the distance 3 times because, one-time reduction in distance will have twice effects on the charges, as distance contain square on it.
