You need to use the most common oxidation numbers for the two elements.
Al: 3+
S: 2-, 2+,4+and 6+ (2+ is rarer than 4+ and 6+).
Son you can predict a compound made up with Al 3+ and S 2-
=> Al2 S3 (remember that you exchange the oxidation numbers).
Al2 S3 is named aluminium sulfide and you can obtain it from this reaction of the elements:
2Al + 3S ---> Al2 S3
Answer: Al2 S3
