Because I studied this stuff already
If a textbook asks you, “How did you know,” there’s really only one answer for it: because I studied this already. What, do they expect you to pull some magical formula out of thin air and hope that it’ll yield the correct answer? It’s time for textbooks to stop asking stupid questions!
The section above pretty much had the student use the same method of adding two figures together, so it’s a bit late to ask how they knew to regroup to use to solve the same kind of problem. Of course, we’re kidding, and this kid should take his education more seriously.