Consider point charges of +Q and +4Q, which are separated by 3.0 m. At what point, on a line between the two charges, would it be possible to place a charge of -Q such that the electrostatic force acting on it would be zero?

The law of conservation of electric charge: The net amount of electric charge produced in any process is zero. or
No net electric charge can be created or destroyed.

Three properties of conductors in static equilibrium.
1. The electric field inside a conductor is zero in the static situation.
2. Any net charge on a conductor distributes itself on the surface.
3. The electric field is always perpendicular to the surface outside of a conductor.

Gauss's law: The total flux summed over any closed surface is equal to the net charge enclosed by the surface divided by eo.