I have written a book that spoon-feeds the student in getting up to speed on the centroid of a triangle. The book is available on Amazon.
The title of the book is:
Locating the Centroid of a Triangle
Although the book is consumable, I suggest keeping it blank and just photocopying the pages that you want to use. I have put the book in the Public Domain, so such copying, even for financial purposes, is no problem.
Here is the link to the book on Amazon: