So SO4 2- has to add up to its charge, negative one so the oxidation number of oxygen in a compound is always -2 multiply this by 4 and you get -8, meaning the charge for S has to be +6 for its charge. Since it’s a compound with aluminum, Al’s oxidation number is +1 as there is two cancelling out and balancing it out to zero
Here’s a recap: Al is +1
Sulfur: +6
Oxygen: -2