What college rules would there be about posting a math homework exercise with one's work from a math class in the Math.SE?
In most cases, using an online Q&A site to do your homework for you would be cheating. Consequences range from 0 credit to expulsion.
Even if you aren't caught, or if homework isn't typically graded where you are so cheating isn't an issue, assignments are typically designed to help you learn, and having someone else do your work is cheating yourself out of learning.
It may be possible to ask legitimate questions that help you understand concepts rather than actually doing your work for you, but first try to do your best from the course materials and ask only when you are truly stuck.