Since the given formula is . According to cross method formula, magnesium has +2 charge so, is multiplied by 2.
Thus, 1 molecule of magnesium phosphate will contain 2 atoms of phosphorus.
Therefore, three molecules of magnesium phosphate contains following number of atoms.
Mg = 9
P = 6
O = 24
Hence, we can conclude that there are 6 atoms of phosphorus in three molecules of magnesium phosphate, .