An author receives $0.75 for each hardcover book or paperback book that is sold. There were x hardcover books and 42,000 paperback books sold of her most recent book. The author received a total of $60,000 for the book sales. The equation below can be used to determine the number of hardcover books that were sold. 0.75(x + 42,000) = 60,000. How many hardcover books were sold?