Find the difference between a mountain that has an altitude of 3486 feet and a valley that is 139 feet below sea level.

Answer 1

The altitude of the mountain is its positive height ABOVE sea level (3,486 ft)

The depth of the valley is its negative distance BELOW seal level (-139 ft)

The difference between the two heights is 3,486 ft - (-139 ft) or 3,486 ft + 139 ft = 3,625 feet

(Using subtraction of signed integers rules)