Nitrogen and hydrogen gases react to form ammonia gas via the following reaction:
n2(g)+3h2(g)→2nh3(g)
at a certain temperature and pressure, 1.9 l of n2 reacts with 5.7 l of h2.

if all the n2 and h2 are consumed, what volume of nh3, at the same temperature and pressure, will be produced?

1mol N2 + 3mol H2 → 2mol NH3 
Because you are dealing with gases, at the same temperature and pressure you can say: 
1volume N2 + 3 volumes H2 → 2 volumes NH3 
OR: 4 volumes reactants = 2 volumes products. 
You have 1.9+5.7 = 7.6 volumes reactants, which must give 7.6/2 = 3.8 volumes of NH3 
3.8 litres of NH3 will be formed

The air is going to come out because of the low pressure.