The order of the given reaction has to be determined. How the order depends on the initial pressure has to be explained. How does the mechanism of the reaction vary with pressure has to be explained.
Order of a reaction: The sum of exponents of the concentrations in the rate law for the reaction is said to be order of a reaction.
Mechanism of a reaction: The representation of step by step process involved in the chemical process is said to be mechanism of a chemical reaction.