I see this as a basic equation.
If she starts at point x, 8 feet, then goes down another 13, she is adding to the number of feet she traveled down.
So your answer would be 8+13=21
If this isn't what you needed, because I am not sure what math you are taking, let me know if I can help you more!
We first obtain the equation of the lines bounding R.
For the line with points (0, 0) and (8, 1), the equation is given by:

For the line with points (0, 0) and (1, 8), the equation is given by:

For the line with points (8, 1) and (1, 8), the equation is given by:

The Jacobian determinant is given by

The integrand x - 3y is transformed as 8u + v - 3(u + 8v) = 8u + v - 3u - 24v = 5u - 23v
Therefore, the integration is given by:
To do this, you can just multiply across. 3 times 1 equals 3 so your numerator is 3. 2 times 4 equals 8. Then, your answer would be 3/8. Hope this helps :)
550+450-120+720=1,600
720/1,600*100%
=720/160%
=45%
therefore, 45% of new balance