Amersham station is the highest station in the london tube at 147 meter above sea level. waterloo station is the lowest station at 21 meters below sea level. what is the height difference?

Johnny rode his bike to a friends house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was unable to play. what was his displacement for the total bike ride trip? what could you use as a reference point ? show the steps to solve this problem.