##Mg_3(PO_4)_2##

Magnesium is in the group 2 of that means that it’s number of oxidation is 2+. Phosphate formula is ##PO_4^(3-)##, also we can see in the lewis structure that it has a charge of 3-. So to balance the charges Mg +2, and ##PO_4## -3, the easiest way is to multiply between them, that is ##+2*3=+6##, and ##-3*2=-6##, and now the charges are equal. Meaning you need 3 atoms of ##Mg^(2+)## and 2 molecules of ##PO_4^(3-)##, for charges to be equal.

##Mg_3(PO_4)_2##

Magnesium is in the group 2 of that means that it’s number of oxidation is 2+. Phosphate formula is ##PO_4^(3-)##, also we can see in the lewis structure that it has a charge of 3-. So to balance the charges Mg +2, and ##PO_4## -3, the easiest way is to multiply between them, that is ##+2*3=+6##, and ##-3*2=-6##, and now the charges are equal. Meaning you need 3 atoms of ##Mg^(2+)## and 2 molecules of ##PO_4^(3-)##, for charges to be equal.