Question: two metal spheres each of mass 015 g are suspended...

Two metal spheres, each of mass 0.15 g, are suspended by massless strings from a common pivot point at the ceiling, as shown in the figure. When both spheres carry the same electric charge, we find that they come to an equilibrium when each string is at an angle of θ = 4.0° with the vertical. If each string is 21 cm long, what is the amount of the charge on each sphere? (Enter the magnitude in nC).