A2 Chemistry - AQA unit 4 - rate determining step?!

I've spent a ridiculously long time trying to understand how to determine which step is the rate determining step in a multi step reaction. for the question below, the rate determining step is step 2... but why is this!?!? I don't understand :


In the presence of the catalyst rhodium, the reaction between NO and H2 occurs according to the following equation.
2NO(g) + 2H2(g) ---> N2(g) + 2H2O(g)
The kinetics of the reaction were investigated and the rate equation was found to be rate = k[NO]2

Using the rate equation and the overall equation, the following three-step mechanism for the reaction was suggested. X and Y are intermediate species.


Step 1 NO + NO ---> X
Step 2 X + H2 ---> Y
Step 3 Y + H2 ---> N2 + 2H2O

Suggest which one of the three steps is the rate-determining step.

Explain your answer

(This is a question obtained from June 2011 AQA A2 Chem 4)

thanks in advance!