Great question!
There is no exception for exercise material. The source citation will immediately follow 3b with no blank line.
Begin the course citation in cell 9 rather than cell 10. I know it can be confusing. Formatting is almost always an odd-numbered cell. When it states that sources begin in the 5th cell to the right of the previous line, that does not mean 5 blank cells. In your sample, cells 5-8 are blank and cell 9 is the 5th cell.
Look at the cell count before each item below:
1-2-b. text text text text text (3-5)
1-2-3-4-runover text runover text. (cell 5 for the runover)
1-2-3-4-5-6-7-8-Source (9-9)
I hope that helps.
Thanks for your question.