There can be two answer choices. As it doesn't specify which way the guest travels the 15 floors, it can go up, or go down.
If the elevator goes up:
Add the two numbers together
-6 + 15 = 15 - 6 = 9
9 is your answer
If the elevator goes down:
Subtract the two numbers
-6 - 15 = -21
-21 is your answer
hope this helps
