Answer: Ammonium, when heated with aqueous base, will give off NH3 (ammonia) gas, (and depending, water vapor). This will leave the Cr2O3(s). From then on,
it is just adding or subtraction of gases or water vapor. You probably heard “Loss of electrons is Oxidation”, “Gain of Electrons is reduction”. That should help.
Explanation: This isn’t an explanation but an interesting point; Acid-Base and RedOx reactions are useful to the most complex of any Chemistry. Get this down, and Organic Chemistry will be much easier.