This is a topic that have been discussed several times during the years. Everytime the community have agreed on how it should be done, but it would be good that it was communicated to newcommers.

Follow the rules of Flagging

If you know you have a clear and complete answer, and you are certain you can follow up any comments, it is ok to Answer

In other situations make a comment so that others can fill in, or make an Answer.

The requester should answer when the comments are good enough, stating that the issue was solved, and perhaps how.

If the requester doesn't answer, but responds in a comment that it was resolved, anybody else can answer with "Answered in comments" or such. Or the one that gave the answer, can copy it to be an answer.

If there haven't been activity for over a week (in case it is a weekly lesson) and it seems there is a proper answer it can also be closed, asking the requester to open a new if there is need.
If there doesn't seem to be a good answer, and there is no activity, you can decide. Ask the requester. Check the code if it seems to be fixed. Answer it with "Closing request, open new if needed"