applied problem: about the difference between elevations

Question

an airplane flies at an altitude of 14,690ft while a submarine is submerged at a depth of 521ft below the surface of the ocean. What is the difference between these two elevations?  how do you  write an algebraic expression?

Answer 1

To determine distance take ending point and subtract beginning.

If we start at the plane, 14690 - (-521) = ?