Drills cannot be re-done, but you can always review your results and often do more drills.
We always recommend going back to review anything that you missed our guessed on. When you're on the Homework page, just select Review to see your results.
You can toggle between the answers by clicking on the bubbles near the top of the page. The explanations will be at the bottom, and the correct answer will display in green.
While Quizzes offer one set of questions, you will often have the option to Do More drills after you complete a drill the first time. Some drills even let you select this option to do more up to 5 times!