An atom consists of electrons, protons, and neutrons. Each electron has a charge of -1, each proton has a charge of +1, and each neutron has no charge. If 3 electrons are removed from each of 4 atoms, what is the combined net change to the charge of the 4 atoms? Three electrons are removed from each of 4 atoms. Write an expression to represent the net change on the charge for the group of atoms.