Earlier quoted context omitted.
Like when you solve the arithmetic problem with algebra and the teacher says it's wrong because you didn't use the official method?
on the test that's testing whether you're proficent with _official method_? or maybe because your _official method_ was "under some assumptions" or "by guessing that" I've heard that complain countless times from friends and I think always the reason was that it wasn't formally correct.
Especially with something broad like algebra, there are often many ways to solve a problem.