So,
We know that Rachel earns 51 dollars each week.
We know that She earns one additional dollar for each book that she sells. So, we can write:

Where A is the amount of dollars and B is the number of books She sells in a week, which is equal to the number extra dollars that She earns.
If she sells 23 books, then She earns:

She earns 74 dollars if she sells 23 books in a week.