Hello!! In order for your equation to be balanced, you need to have all things equal the same thing on each side. Therefore, your equation would read:
3 S(sub 8) + 12 O(sub 2) -> 8 SO(sub 3)
This would allow everything to equal 24.
I hope this has helped you and if you need any more questions answered with balancing, let me know. Have a great day!!
