2nan3=2na+3n2. if 0.21 moles of nan3 reacts, what mass of nitrogen would result

Use the mole ratio:  2 moles of NaN3 :  3 moles of N2
             you have :   0.21 moles of NaN3  : x moles N2
             Cross multiply: you get 0.3 moles for N2
To find the mass= # of moles * molecular mass
                             0.3 * 28 
                       = 8.4g