1. Last year, a company sold 15 000 copies of a book, at a price of £25 each.

This year, the company wants to increase the price of the book and predicts that for every £2.50 increase in price, annual sales will drop by 750 books.

The formula $N=a+bc$ is used to model the number of books sold annually. Where $N$ is the number of books sold, $c$ is selling price of the book and $a$ and $b$ are constants.

Find the values of $a$and $b$and hence, write down the model used for the number of books sold annually.