Is there a way to delete questions that you have found the answer to?
You could consider leaving them around.
For starters, regular contributors don't have the ability to delete questions, so your go-to move here will be to get a moderator to do that for you.
That said, you should consider leaving the questions around until they get answered. Even if you already found the answer to a particular question, having it around can be helpful to other students.
In fact, you could post the answer yourself, if you want. That way students who are searching for an answer to the same question won't have to ask it again.