Three charges q, q and q are placed at the vertices of an equilateral triangle of side l as shown in the figure. if the net electrostatic energy of the system is zero, then q is equal to

 Electric potential energy is possessed by an object by the virtue of two elements, those being, the charge possessed by an object itself and the relative position of an object with respect to other electrically charged objects. The magnitude of electric potential depends on the amount of work done in moving the object from one point to another against the electric field.Potential is zero means system is in equilibrium.

E = 2kQq/r + kq2/r=0

Q = −q/2