Chemistry, How would you approach 3?

3.
The order of overall reaction is equal to the molecularity (number of molecules appearing on the reactant side) of the rate determining step.

a. 1 molecule (SO₂Cl₂) appears on the reactant side, i.e. first order
b. 3 molecules (2NO₂ + O₂) appear on the reactant side, i.e. third order
c. 2 molecules (2N₂O) appear on the reactant side, i.e. second order
d. 2 molecules (2HI) appear on the reactant side, i.e. second order

The answer: b. 2NO(g) + O₂(g) → 2NO₂(g)