Entropy help? ASAP please? due tomorrow?

Calculate change of S for the reaction:
2NO+ O2 =2NO2
values for each in J/mol*K

The equation to solve the problem is as follows:
Change Srxn.=(sum of the products*n*change of Sf)-(sum of the reactants*n*change of Sf)

I don't know what n stands for, please help?
Update: Thank you all- now can you tell me what change of Sf is? I know that it is probably a lot simpler than I am making it...
Update 2: Thank you so much Dr. Lee!
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