What I would do is:
Evaluate the source. Is it from your own head/thoughts? Is it from your notes the teacher gave you? If it is, don't cite it.
Is the answer or source NOT from you and your own knowlege? Is it something that came from an external source, such as a book or website, cite it.
Better to cite than be accused of plagarism, so if you are unsure, I would cite it just to be safe.