24-11+8
He BOUGHT 24 stickers, so you'd write 24. Then because he GAVE AWAY 11 stickers, you would subtract 11 from 24, which is 24-11. Then because Al BOUGHT 8 stickers you'd add the 8 to the 24-11 to get 24-11+8.
(Bought means to add and Gave away is subtracting)