This is a pretty uncommon scenario here, but it appears that someone has posted what looks a lot like a homework question to the site. The twist here is that the question itself is very relevant to the site, and if the OP would have put any effort into modifying the question to appear as if it was their own, we would never have noticed. Is there any issue with leaving this question open?
Personally, I say leave it open.
- There's really no way to police this.
- The question is interesting; removing it punishes ourselves.
- This happens so rarely that doesn't really need a policy.
EDIT: I posted this before @ff524 found the exact question, along with answers, already online. Given that update, I'm a bit more hesitant... as @Strongbad says, we don't need to repost stuff from other sites just for the heck of it.