To solve this one we will subtract how much he spends from what he has. At the start he had 19 and then spent 8 meaning he now has 8 less than when we started so 19-8 is 11. He now as 11 dollars, then he spend another 4 meaning we have to subtract another 4 dollars. 11-4 is 7. So he as 7 dollars left at the end.
I hope this helps and please don't hesitate to ask if there is anything still unclear!
