In order to find the mass of an element, you can simply go on the periodic table and find the element you need. You need to do stoichiometry using the ratio. the amount of grams NaN3 have and convert it over using the molar mass. That way you can get moles only and then use mole ratio. For every one mole of NaN3, there's one mole of Na. After that, convert the moles to grams again to get the final amount of Na is produced.