Anitrogen oxide is 63.65% by mass nitrogen. the molecular formula could be

To answer this question you need to know the molecular mass for both atoms. Nitrogen molecular mass is 14. Oxygen molecular mass is 16. If the 63.65% mass is nitrogen, then the 36.35% would be oxygen.

Then the ratio of nitrogen/oxygen would be: (63.65% /14) / (36.35% /14)= 4.53 /2.28 = 2
That mean there will be 2 nitrogen for every 1 oxygen. The molecular formula should be N2O