because you can replace your final result following rule one

The expectations on this one were baffling, as the description was poorly written and left out several important things that would actually be tested.

I think it should have had better sample tests, clearer descriptions of which variable needs to be returned, and actually stated things that might not necessarily be obvious, like how all coefficients will be integers, that variables could be both uppercase and lowercase letters, and that formula might have only one variable.

I solved it, but I hate it.