Don't feel confused, take it slowly and it will work out eventually.
Mass number(Ar) is the number on top of the element on the periodic table. Remember the subscript of the chemical formula means how many atoms of that element per molecule. So getting molar mass(Mr) of the compound you have to multiply the element mass number(Ar) by the subscript integer. Once you get the molar mass, just apply the formula to get the moles.