Earlier, you did the Roots of a quadratic exercise. If you haven't done it, do it now.
Rewrite it, adding error checking and Subs. It should work like this:
Some of the code has been written for you. Download the workbook. You'll see code like this:
Upload your solution.
In the text part of the submission form, please answer the question: What did you learn from this exercise?
(If you were logged in as a student, you could submit an exercise solution, and get some feedback.)