1) A homework problem (with book citation)
Excellent. This provides part of the context, namely where the problem came from. Sadly, many other people don't even bother to state this, and is one possible factor for closure.
2) The inquirer's workings towards an (ultimately incorrect) final answer
Great. As long as it is readable, it is perfect.
3) The textbook's answer
This is usually not needed if you're asking about mistakes in your solution, unless one's question involves the textbook's solution, such as:
Why did they ..., when it seems I can do it another way (or omit it altogether)? Is my reasoning correct?
I don't understand this step in the textbook solution: ... It seems similar to the part in my solution where I ..., but their explanation seems to be different.
Whether you provide this or not, it should not be a reason for closure.
4) And lastly, requests help identifying where their math is incorrect? They do not explicitly request a solution.
Current consensus is that such questions are on-topic at Math SE, and almost always far better than those URGENT-PLEASE-SOLVE questions. As mentioned by Martin Sleziak, these questions should be tagged proof-verification or solution-verification or both. For specific questions you will have to provide a link to the question if you want reasons for its closure discussed.
5) What if the homework question was posted already on stack exchange and solved a year ago?
This is usually not relevant, because the question is about a particular attempt at a solution. "What is the mistake in my solution?" is very different from "What is a correct solution?" and sometimes looking at correct solutions is not enough to tell you what you did wrong (unless you've a full grasp of logic, in which case you probably will never need to ask where your mistake is since you can find it yourself).
It may be possible that someone has posted the exact same faulty solution as yours before, in which case it would actually be a duplicate question and can be closed, but still no reason for downvoting.