If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers is law of multiple proportion.
Nitrous oxide formula is N2O and Nitric oxide has the formula of NO
The ratio of nitrogen to oxygen in the first case is 2 : 1 and in the second case its 1 : 1.
That means for the fixed amount of oxygen nitrogen combine in the ratio of 2 : 1. This indicates the law of multiple proportion.