For every action you want to record the "Distance", you do not want to find the displacement. 210 feet + 300 feet = 510 feet. His total distance traveled is 510 feet
Displacement however would be -90 = 210 - 300 because he is now -90 feet from where he begun. The reason the 90 is negative is because he is lower now than he was before because the downward movement was greater than the upward movement.
