Tyler Wallace's "Beginning and Intermediate Algebra: Section 6.5: Factoring - Factoring Special Products"
Read this section. There is an error in Exercise 308 on page 231. You can find the correction in Saylor Academy's "Correction to Exercise 308, Page 231".
Click https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2011/12/SAYLOR-MA001-TEXT.pdf link to open resource.