What compound forms when ammonium cations combine with nitrate anions?
The trick here is to realize that you're dealing with two polyatomic ions.
The ammonium cation,
Now, an ionic compound, which is what you get when the electrostatic force of attraction pulls a cation (a positively charged ion) and an anion (a negatively charged ion) together, must be neutral.
This implies that the total positive charge coming from the cation must be balanced by the overall negative charge coming from the anion.
In your case, the ammonium cation carries a
#["NH"_4]^(+) + ["NO"_3]^(-) -> ["NH"_4]["NO"_3]#