Start at zero on your number line.
To add 15 floors, we count up to 15. (0+15=15)
Then we go down 9 floors...count 9 down and you'll get to 6. (15-9=6)
Then we go up 11 floors...count up 11 and you'll get to 17. (6+11=17)
Then down 19 floors...count down 19 and you end up at -2. (17-19=-2)
The net change is 2 floors down.
