Should I delete my question on Stack Exchange to reduce my own plagiarism?
From what you have reported, it does not sound like you have committed plagiarism, i.e., you have not claimed the work of anyone else as your own work. So you don't need to worry about that---and if other people steal your work, that is their fault and not yours.
You might, however, also be concerned that you have broken a class policy on obtaining outside help, and that you might be exposed by others plagiarizing you. If you have not broken class policy, then don't worry about it. If you have broken class policy, then 1) don't do it again in the future, and 2) trying to hide it probably won't help you, since this site gets widely copied and cached in any case.
You asked the question, and you received the answer originally. Other people copying from the same question should not be a problem to you.
What you should do when receiving help online though is making clear to whoever does the grading that part of the exercise was solved using help by someone else (by citing them/linking it).
In case there will be an aftermath, you will be able to prove you were the one asking that question, since it is linked to your account.
Depending on class you did the exercise for, you might get a worse grade when doing so, but it is the right thing to do.
OK. Sounds like a bit of an interesting situation you got here. If a classmate turns that answer in before you, your best option here would be to explain to the teacher your situation. You posted a question on a well-known site, and it received a lot of attention, eventually attracting some of your classmates, who probably didn't even know you posted it. In this case, they have (most likely unknowingly) committed plagiarism, as defined here:
Plagiarism:
To steal and pass off (the ideas or words of another) as one's own
to use (another's production) without crediting the source
- to commit literary theft
- to present as new and original an idea or product derived from an existing source - Plagiarism.org
If this happens, just set up a time to talk with your teacher and calmly and clearly explain that to them. If they are a reasonable teacher, they will understand and you can go from there. Good luck!