What is the difference between a mountain that has an altitude of 3738 feet and a desert valley that is 491 feet below sea level?

Answer 1

If you are asking the distance between the top of the mountain and the bottom of the valley, that would be

3738 + 491 = 4,229 ft

Since sea level is considered the zero level (kinda like the origin of the Cartesian coordinate system), the mountain is the + sid and the valley is the - side so +3738 - (-491) = 4,229