The correct answer to this question is 65,000 feet.To calculate the difference between elevations below sea level and above sea level, you add the elevations together. In this case, we are finding the difference between the Earth's highest elevation and the lowest known elevation.
To get a quick estimate of the difference, round the elevation of the Mariana Trench from 35,840 feet to 36,000 feet and round the elevation of Mount Everest from 29,028 feet to 29,000, and then add the estimates together. 36,000 feet + 29,000 feet = 65,000 feet. Even though the Mariana Trench is below sea level (-35,840 feet), you use positive values to find the difference between two points.