You can absolutely puzzle out _a_ solution to that problem, it's not just a quiz. It's not even hard, given the context that you know that it's possible to do.

Steps, mostly driven by just basically knowing the goal and that there's not many operations that could possibly help:

"a" "b"

"a+b" "b"

"a+b" "-a"

"b" "-a"

"b" "a"

Then once you have that, you can enumerate the downsides to that, look for more efficient and less error-prone ways to proceed.