The airplane is at an altidute of 14,690 ft while the submarine is submerged 521 ft.
This means that the airplane is at +14,690 and the submarine is at -521.
To find the difference, we subtract:
14,690 - (-521) =
Notice how we have two negative signs next to each other, this makes a possitiv
14,690 + 521 = 15,211
The difference between the two elevations is 15,211 ft
I hope this helped!