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.