first: mg sin30=q2 /4??0l2 at equilibrium.
as the component of gravitational force is balanced by the electrostatic repulsion.
here the length between the centre of the two balls is 2l sin 30 =l from simple geomety,see by drawing the diagram.
now at a point which is at 2:3 dist, resultant intensity=(1/4??0)q(25/4l2 -25/9l2 )
putting the value of q from the first equation of equilibrium)=approximately 43.3×103N