Okay, so you need to start by finding the molar mass (grams in one mole) of nitrogen monoxide (NO).
N=14g
O=16g
So we know that NO has a molar mass of 30 grams, then just divide your given mass of 22.5 grams by the molar mass of 30 grams to find the number of molecules in your sample. The answer should be .75 molecules. Hope this helps!