If this is the salt formed in a double displacement reaction (neutralization), what were the reactant(s)? Ra3(PO4)2

Sally is making a model of a magnesium atom with an atomic mass number of 24 for her chemistry class. she has foam balls for the protons, neutrons, and electrons. she has added 6 neutrons to her model so far. how many more neutrons does she need to add to complete her neutral atom of magnesium?