Ferric Bromide + Sulfuric Acid = Mikasaite + Hydrobromic Acid
2 moles of aqueous Ferric Bromide [FeBr3] react with 3 moles of aqueous Sulfuric Acid [H2SO4] to form 1 mole of aqueous Mikasaite [Fe2(SO4)3] and 6 moles of aqueous Hydrobromic Acid [HBr].

Therefore, the balance would be 2FeBr3 + 3H2SO4 → Fe2(SO4)3 + 6HBr