The formula must be written
Ionic compounds combine positively charged ions (cations) with negatively charged ions (anions).
The resulting compound will be electrically neutral - that means the compound has an overall charge of zero.
The ammonium ion (
This video discusses how to determine the chemical formula for some additional examples of ternary ionic compounds. The video discusses how to write formulas for; lithium hydroxide, magnesium nitrate and aluminum sulfate.
Hope this helps!