Im studying for the MCAT and i found this question in my old chem book. i can't figure out how to derive the formula they are talking about...

Question

Nitrogen monoxide reacts with oxygen to give nitrogen dioxide.

2 NO(g)+ O2(g) --> 2 NO2(g)


The rate law is -[NO]/t = k[NO]2
[O2], where the rate constant is 1.16 x 10-5 M^-2∙s^-1at 339oC.

The initial pressures of NO and O2 are 155 mmHg and 345 mmHg, respectively. What is the rate of decrease of partial pressure of NO in mmHg per second?

(Hint: From the ideal gas law, obtain an expression for the molar concentration (mol/L) of a particular gas in terms of its partial
pressure.)