Using the relationship of density (ρ) of the substance at temperature (T)

We have ρ = Mp/RT

or p = ρ RT/ M

For the given data if M is the molar mass of the gaseous oxide, we have

2 = ρ RT/ M ……………(1)

Also for nitrogen 5 = ρ RT/28 ………………….(2)

From (1) & (2), we have

5/2 = M/28

Or

M = 5 X 28/ 2 = 70g/mol

Hence, the molecular mass of the oxide is 70 g/mol.