left: 1 iron, 3 bromine, 2 hydrogen, 4 sulfate
right: 2 iron, 3 sulfate, 1 bromine, 1 hydrogen
you need 2 FeBr{3} to get 2 iron on the right, and you need 3 H{2}SO{4} to get 3 SO{4} on the right, and that will also give you 6 HBr
so 2 FeBr{3} + 3H{2}SO{4} --> Fe{2}(3SO{4}) + 6HBr