Dear student ,
The number of moles of sodium nitrate = 1010-3 = 10-2 moles = 0.01 moles
Again the number of moles of NO2 = = 0.167 moles
Total number of moles of N = 0.01 + 0.167 = 0.177 moles or 0.1776.021023 atoms = 1.06551023 number of atoms
Again the total number of moles of O = 30.01 + 20.167 = 0.364 moles or 0.3646.021023 atoms
= 2.191023 number of atoms
Regards