Hello Wendy,
You cannot do these things after the quiz has been attempted. This applies to all quizzes which include all question types, not just Formulas questions.
If the quiz has already been submitted to students, it's too late and there's nothing you can do.
If the quiz has not been submitted to the students, you or someone else has taken the quiz with a student role. You just have to delete the attempts (all of them). Click on the name of the quiz, then on "Attempts: xx" in the center of the page. Select all attempts and delete them. But don't do this if they are real student attempts!
By the way, very nice work!