If you know the correct answer to a question, you should post it — whether or not you are the original poster (OP) of the question. This is true whether you know the answer from the start (i.e., at the time you posted the question) or learned it later.
Along the same lines, users are encouraged to (1) add questions that they know the answer to and then (2) answer these questions immediately.
Think about Quora as an accumulating database of knowledge. It's good to ask and answer questions you know the answer to, because once you put your knowledge into the system, other people can draw from it. If you add the answer and/or do the research yourself, subsequent users with the same question can get the answer with less effort. On Quora, it's not unusual for different people to be serendipitously interested in the same question. If you can share the work you've done of researching the answer, then that's a win for everyone.