Share 'help with les articles'

Hi, I recently received my submitted work back from my tutor and she underlined with a big red cross "chapitre trois" or "chapitre X". I need to refer to the chapters in the book as a reference for my arguments, so often I would say "selon chapitre x" or "comme l'a indiqué par chapitre x". Is there a problem with saying "chapitre X"? is there supposed to be an article or something before hand? H…