I'm with Bobby on this one, you should write the problem up in your own words. I think it's important to point out, though, that the answer to this question changes depending on the other site's rules.
For example, if the other site is CC-BY, then go ahead and reuse with proper attribution. Other sites do this with SO's content (some of them following the rules, others not; check out the
clone tag for more on that).
On the other hand, if the other site says that all of its content is copyrighted, then I'd write the problem up completely independently and just add a link that says "there's discussion about a similar problem here, but it doesn't reach a definitive answer."
Found the related link I was thinking of this morning. There was a specific case of this happening with MSDN not too long ago. Check out the comments and Marc's answer.