If you are the answerer of the original answer, you are free to re-post your answer on other appropriate questions on other sites, as long as it's truly a good answer. If you copy/paste it directly, it will trip an automatic moderator flag, so be sure it is truly a good answer, and not something that smells like spam.
If you are not the original answerer, and the question is on-topic for the site in question, you are free to post the answer with attribution, ie
As Jon Skeet posted in this Stack Overflow
question,
the time zone changed in Shanghai in 1927 causing unexpected results
when doing math across that period.
Now, technically speaking, if you do not reproduce the answer exactly, but read the answer, commingle it with information you already have, and produce de novo a new answer that is similar, but not identical, that does not require attribution - that is fair use, so long as the new answer has sufficient differences. (It's not like we expect every answer to have an attribution, and everyone learned their information from somewhere.) However, it's still polite and appropriate to attribute it if there was a fairly direct translation from the old answer to your new answer (and, it's much better for the SE network as a whole, since it gives a good link between related points).
If the question is not on topic for the site in question, I would vote to close if it has no bounty, or at least note that it is not on topic for that forum in a comment. Either way, I would then post a link to the question and answer on the appropriate site in a comment (but nothing beyond that - just 'Here is the right place to ask that question, and it has a good answer.')
This is only appropriate if it is off topic for the second site; if it is on topic for the second site, and it is at least technically a distinct question (ie, it's not the same person crossposting on both sites), then it's appropriate for it to be separately represented on both sites.
