Homework is assigned, submitted, and graded via WebAssign. You must purchase an access code for WebAssign in order to use it. If you bought the course text from the UW Bookstore or bought the eBook, both should have come with an access code. If you don't want the physical textbook, the cheapest option is to purchase the eBook/access code combination.
If you've never used WebAssign before, a guide is available here.