This question just asks for people to solve some short cipher. I don't think the question is particularly "mathematical", and I would say it's off-topic as posed.
I don't like this question either.
It's a puzzle, not containing (as far as I can see) anything of genuine mathematical interest. I don't object to questions about the mathematics behind cryptographic methods, but this is not such a question.
It's asked poorly. The OP has at least admitted it's homework, but not indicated what progress (s)he has made on it.
So, I have voted to close.
Cryptanalysis is mathematical and, if that is not enough to have it considered as on-topic, is posted rarely enough to err on the side of not closing.
Adding sites for puzzles and/or cryptography is a better long-term solution than excluding cryptograms as "not (100 percent) mathematics".
There are also rating and tagging mechanisms in place. The question has, so far, 5 downvotes and 0 upvotes. The [homework] tag will not make it more popular or active. Such postings drop in visibility. Why this one posting needs to specifically be excluded is not clear.
Debating individual questions one by one is less efficient than developing generic solutions that apply to all postings, such as introducing a [cryptanalysis] tag or refining the visibility metrics.