In this problem we're looking at the formula for a compound formed from ammonium and phosphate. So we have two polyatomic ions. So just like we were looking at monatomic ions, the first we want to do is write our formula for each of the ions. So we have NH4, which has a plus 1 charge. And phosphate, PO4 3 minus. And so I'm going to do the exact same thing that I did with monoatomic ions, which is crisscross down to find my subscripts. And what I find is I get NH4, and I need three of these, so I am going to have to put this in parentheses. Because I need to make sure I say I have three ammonium ions, and not say 3 times 4 hydrogen, or something like that. So three ammonium ions in PO4. Because my subscript on the phosphate is only going to be 1, because the charge on the ammonium is 1, I don't need to include parentheses around the phosphate ion. They only need to be included on the ammonium ion. When I look at my charges, I see that I have 3 times the 1 plus of the ammonium, plus the 1 times the minus 3 charge on the phosphate, and I see that again, the sum of the charges of the ions in this compound sums to 0, which is exactly what I want for a neutral compound.