Answer:
Step-by-step explanation:
Let
h-------> the minimum height for riding on the Super Screaming Spinner at the theme park
we know that
the height may not be less than 
so
-------> inequality that represent the situation
Shirley/Tracey has to stay at least 10 days for the M and N to cost less
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!