An electric sign that showed the speed

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?

We just started the introduction into circles and i have no idea how to do this.

2x minus y equals 6, x plus y equals negative 3