After retiring, cheryl decided to read every book on a
list of great science-fiction novels. before beginning,
she had read none of the books on the list. once she
started reading, the number of books on the list that
she had yet to read could be modeled
by y = 168 - 2x, where x is the number of weeks
since she started reading and y is the number of books
on the list that she had yet to read. in the equation,
what are the meanings of the numbers 2 and 168 ?