The first point charge was placed at point A, and it created a potential 2B at point B. Then the first charge

The first point charge was placed at point A, and it created a potential 2B at point B. Then the first charge was removed, and a second point charge was placed at point B. He created a potential of 9B at point A. Then the first charge was returned back to point A. With what force do these charges interact?

The force of interaction of two charges q1 and q2 is determined from the Coulomb law:
F = k * q1 * q2 / r ^ 2,
where
k = 9 * 10 ^ 9 H * m ^ 2 / Kl ^ 2 – constant,
r is the distance between point charges;
The charge potential q1 at point B is determined from the formula:
φ1 = k * q1 / r = 2B,
and the charge potential q2 at point A:
φ2 = k * q2 / r = 9V;
From these formulas you can get:
F = φ1 * φ2 / k = 18 / (9 * 10 ^ 9) = 2 * 10 ^ -9H.