I have version 7 and you must of done the homework incorrectly if you used chapter 6 to answer the question for module 6.
Chapter 6 discusses returns to scale on page 156. It relates increasing, constant, and decreasing returns to scale when applied to the long run of cost curves.
The homework question specifically requests us to relate constant cost and decreasing cost industry. This relates to economies of scale not to the returns of scale.
The main difference is that returns to scale relate to increasing "all" of the inputs which can effect the output of increasing, constant, or decreasing given one individual company.
Whereas, constant-cost and decreasing-cost industry relate to a whole industry and how their cost's are distributed.
These are very, very different subjects. I suggest you revisit the homework once you have read Chapter 7.
Sorry but I would rather have you mad at me but still get the homework correct and the opposite!