Sabah has $450 to pay for college textbooks. She expects to pay about $75 per book. Her friend told her that 4 of them can be checked out of the library for free.

Complete the equation below to find the total number of books that Sabah can get for her classes. Use b to represent the total books.

The equation has to look like this format: ?(?-?)=?
You can use any of these numbers to fill in the equation: 4, 10, 75, 300, 450, b