Hello Xia,
Thank you for your question.
To answer your question.
ln 6: Cell five heading. Blank line before but not after. (BF.4.5)
ln7-ln10: This text is directions. Directions are formatted in 5-5. (BF10.3)
ln12: Cell seven heading. Blank line before but not after. (BF.4.6)
ln13-ln14: Exercise material with sub-entry formatted in 1-5. (BF10.4)
ln15: Sub-entry formatted in 3-5. (BF10.4)
ln16: Blank line required after exercise examples. (BF10.8)
Mike