The equation of the regression line of y (retail price, £) on x (number of pages) for a set of books is y = 0.02x + 1.35. How can I use this equation?

A regression line comes from the line of best fit, y=mx+c. Given your data, you can find the line. C is the point where it crosses the y axis, and m is the slope. Here, we know that the price of a book is y, and x is the number of pages. So 0.02 means that for every page in the book, the price increases by 2p, and there is a fixed £1.35 added to this.