9. The figure shows three identical conducting spheres with these initial charges: A, -12Q, B, +8Q, C, 0. Here is the procedure for connecting and then disconnecting spheres with wire: Step 1: connect A and C and then disconnect； Step 2: connect B and C and then disconnect, moving C far away； Step 3: connect A and B and then disconnect. A and B have a center-to-center separation r. What is the magnitude of the final force between those two spheres? Answer by writing what goes in the blank in this equation: F = _kQ² / r².