Alright so when you're making a chemical formula, you need to know the charges. The roman numeral on a transition metal, such as gold, is its charge. So the charge of gold is 3+. The charge of Cl, chlorine, is 1-. So You need 3 chlorines in order to balance the charges to 0. Therefore your formula will be AuCl3. If you have any more questions, feel free to ask!
