Physics---electrostatics

Let T be the tension of each string, then:
T cos θ = mg
T = mg/cos θ
T sin θ = Q2/4πε0r2
mg tan θ = Q2/4πε0r2
mg sin θ = Q2/4πε0r2 (tan θ ~ sin θ for small θ)
mg (r/2l) = Q2/4πε0r2
r3 = Q2l/πε0mg
r = 3√(Q2l/πε0mg)