You need to strat from the balanced equation for the solution of (NH4)3PO4 in water to give the ions.
(NH4)3PO4 -->
What is the molality of ammonium ions in a 1.75 m solution of (NH4)3PO4?
2 answers
1 (NH4)3PO4 = 3 NH4 + 1 PO4