Katy is hooked on a new book series, The Galaxy. Each book in the series is the same length and chronicles a different year in the Waka Waka Galaxy. Katy has cleared off the top shelf of her bookcase to leave room for each of the books as they come out. There is a proportional relationship between the number of books on the shelf, x, and how much shelf space the books take up (in centimeters), y. The first 2 books in the series take up 8 centimeters of space along the shelf. Write the equation for the relationship between x and y. y = ?