You don't have to worry about converting to moles and back to liters.  You need to know about Gay-Lussac's law of combining volumes. Coupled with Avogadro's law which says that the moles of gas are proportional to the volume, all we have to do is:
2H2(g) + O2(g) -> 2H2O(g) 
2L ..........1L ..........2L 
7.25L .....3.63L .....7.25L 
The only work is dividing 7.25 by 2.