It would be helpful to explain it in terms of moles of coefficients. You can do this through using vapor pressure and number of moles. When the gasses combine, liquid ammonia is formed. Discussing the vapor pressure decreasing as more ammonia forms will likely be the answer your teacher is wanting. There is more pressure and more heat so the energy required to turn the gasses into a liquid is lowered thus creating liquid ammonia and incorporating the ideal gas law would help this proof. This is also a good example of lower IMFs creating a higher vapor pressure of the products. Putting the question in terms of the moles of coefficients would be a good way to utilize Avogadro's law. Since you are lessening the system by 2 moles, a lot more pressure and temperature is required.
P1V1n2T2=P2V2n1T1
I hope this helped. Have a great day. :]