Three identical small spheres of mass 'm' are suspended by threads of negligible masses and length'l' from a common point.A charge'q' is equally b/w the spheres and they come to equilibrium at the corners of a horizontal equilateral triangle of side 'a'.Show that:

q^2=12πεmg a^3 [ l^2-(a^2/3) ]^(-.5)

please help!!