A point charge of +Q is placed at the centroid of an equilateral triangle. When a second charge of +Q is placed at one of the triangle's vertices, an electrostatic force of 4.0 N acts on it. What is the magnitude of the force that acts on the center charge due to a third charge of +Q placed at one of the other vertices? Which of the following is not a vector?"

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.