Careful measurement of the electric field at the surface of a black box indicates that the net outward electric flux through the surface of the box...

Use Gauss's law relate the electric flux with the charge enclosed in the box. Since the electric flux is directly proportional to the net amount of enclosed charge in the box, if the flux is zero, so is the net amount of charge. Keep in mind there may may equal amount of positive and negative charge.

Use Gauss's law relate the electric flux with the charge enclosed in the box. Since the electric flux is directly proportional to the net amount of enclosed charge in the box, if the flux is zero, so is the net amount of charge. Keep in mind there may may equal amount of positive and negative charge.