Neutral metal sphere A, of mass 0.10 kg, hangs from an insulating wire 2.0 m long. An identical metal sphere B, with charge -q, is brought into contact with sphere A. The spheres repel and settle as shown. Calculate the initial Charge on B.

Coulomb's Law Formula = kq1q2/r^2

How do I go about solving this problem? (the book gives 3.9x10^-6C) as the answer)