However, since I did not give the final value of the wavelength, why is this "a complete answer"?
You did give the final value. If the answer is $4$, and I write $2+2$ and leave it unevaluated, I gave the final value. An arithmetical computation is not a meaningful part of solving a problem, and if that is the only step that separates you from the answer, then the problem is done. Sorry but you don't have a case here -- the best you can do is to forget about this and move on.
The comment that ACuriousMind (ACM) left when deleting your question is,
I'm deleting this in accordance with our homework policy. Please do not give complete or near-complete answers to homework-like questions
This comment is basically the same thing that is written in the homework policy under the heading Why don't you provide a complete answer to homework questions?
So even if you wanted to argue that you did not give a complete answer (which I'd agree that AFT's assessment is correct), one could even more easily argue that you gave a near-complete answer that is treated the same as a complete answer to this type of question.