Randomised MySQL Query Validation against another table

I have 3 tables:

1. A list of "users" (uID)
2. A list of "questions" (qID)
3. A "questionsanswered" table which stores which questions have been sent to which users (qaUID and qaQID)

I need to do a randomised query where I select one random question and one random user, BUT not allowed to select previously asked questions so it needs to validate against the questionsanswered table.

Can you help please?