Trina has 72 collectors stamp she puts 43

 So basically what you want to do is use subtraction to subtract those 43 stamps from 72.

72 - 43 basically.

So first, if you want to do it the simple way, subtract 3 from 72. Then you end up with:

69 - 40

And then you can just subtract.

29

So she's left with 29 stamps.