Can someone help me with this picture? (Ignore 45,15)

The head librarian at the library of congress has asked her assistant for an interval estimate of the mean number of books checked out each day. the assistant provides the following interval estimate: from 740 to 920 books per day. what is an efficient, unbiased point estimate of the number of books checked out each day at the library of congress?

Which expressions are equivalent to -6 + 3m

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much