first step
calculate the moles of each element
moles = mass/molar mass
N= 1.78g / 14 g /mol = 0.127 moles
O= 4.05g/16 g/mol = 0.253 moles
then find the mole ratio by dividing with the smallest mole number in each element
that is N= 0.127/0.127= 1mole
O = 0.253/0.127= 2moles
therefore the empirical formula is NO2