how can you calculate the net charge of an ion if you know the number of protons, neutrons, and electrons it contains?

Answer 1

In a neutral atom, the number of electrons ALWAYS is the same as the number of protons. So count up the protons and electrons, subtract, and you have the answer.
Atom A has 20 protons and 20 neutrons. Neutral
Atom A has 20 protons and 22 electrons. Charge is -2 (actually that makes it ion A and we would write it as A^-2).
He^+2 means we have a helium atom (which initially contained 2 protons and 2 electrons and it has lost the 2 electrons.