You bought 12 cards on Monday and 24 on Tuesday. If you add these together, you will have 36 baseball cards in total.
You decided to give some cards (x) to your brother. You now have 13 baseball cards left.
36 - (x) = 13.
36 - 13 = (x).
Therefore, (x) must equal 23 cards given to your brother.
I hope this helps!