The mean that the library and calculated is not correct. Her mistake was she divided 55 by 8 instead of dividing by 10.
Please see the attached picture for the work in finding the correct mean of 5.5 books.
Y= 3.11x+ 17.59
I got this equation by doing 23.81-20.70 to find m, which is 3.11.
Then to find b, or y-intercept, I subtracted 3.11 from 20.70 to get the origin all price and got 17.59