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!
C = 0.3*-2.7 = -0.81
Hope it helps:)
There is 2 midpoints on a line segment.
