Molecular weight of bromine = 159.8 gram
According to Stoichiometry of the reaction given in question
2 moles of Na (sodium ) reacts with 159.8 gram of bromine to produce sodium bromide
1 mole of sodium reacts = 159.8/2 grams of bromine
3 moles of sodium reacts = (159.8/2) 3 grams of bromine
= 239.7 grams of bromine.