.

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