Answer:
The notation "(NH4)2HPO4" indicates that there are two ammonium ions (NH4+) and one hydrogen phosphate ion (HPO4^2-) in the compound. Each ammonium ion contains one nitrogen atom (N), so there are 2 nitrogen atoms in the two ammonium ions. Therefore, the notation "7(NH4)2HPO4" indicates a total of 2 nitrogen (N) atoms in the compound.