Actually, it doesn’t appear that you forgot to mention that–I seem to have missed it. I guess the order in which you wrote it, threw me for a loop.
In that case, I think he should at least give you partial credit. It all depends on what he’s looking for. That’s why I mentioned the lack of context. You’re the one who’s been in the classroom; you’re much better suited than anyone on a message board, to know what your teach. is looking for.
…but context aside, I think your answer works. I just think it could have been clearer.
As a suggestion for the future–just a suggestion–when you get a question, take the question, turn it around into a statement, and finish the statement–thereby answering the question. Applied here:
What was the significance of X?
The significance of X was…
Kind of like algebra.
[edit:] if you’re saying that’s what you did, than never mind–you already know this. I thought you were saying you forgot to mention it, but that it was actually right there in the 2nd clause.