Catherine needs to sell boxes of cookies as a fundraiser for a team. She starts with 135 boxes and begins selling at a constant rate of 15 boxes each day. Use the Segment tool to plot a graph representing the number of boxes of cookies Catherine has left to sell from the time she begins selling until the cookies are gone.

For this case, what you should do is graph the equation of the line that best suits the problem.
 We have then that
 y = mx + b
 She starts with 135 boxes
 b = 135
 begins at a constant rate of 15 boxes each day
 m = -15
 Therefore, the equation of the line is
 y = -15x + 135
 answer
 See attached graphic:
 The cookies are finished on the day number 9
 For that day Catherine has sold 135 boxes of cookies.