Question 1156086
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Same with me: I also do not understand what is they are asking about.


Write to the author of this textbook (or inform your professor), pointing this error:  


he (or she) will be infinitely grateful to you for pointing the error, and, probably, 
will express their gratitude to you by name in the next edition.


Or even will make an offer to you to be their proofreader for the next edition (!)