Final answer:
The force between two charges can be calculated using Coulomb's Law equation F = k * (q1 * q2) / r^2. We can use this equation to find the force between the charges before and after a charge of -3C is given to them.
Step-by-step explanation:
In Coulomb's Law, the force between two charges is given by the equation F = k * (q1 * q2) / r^2, where F is the force, k is the Coulomb constant, q1 and q2 are the charges, and r is the distance between them.
From the given information, we can calculate the value of k using the equation F = k * (q1 * q2) / r^2. Plugging in the values of F = 27N, q1 = 3C, q2 = 9C, and r (distance) = unknown, we can solve for k.
Once we have the value of k, we can use it to solve for the force between the charges when a charge of -3C is given to them. Using the same equation F = k * (q1 * q2) / r^2, we can plug in the new values and solve for the force between the charges.