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?

Given that the highest station in London Tube is at 147 m above sea level and the lowest station is at 21 m below sea level, the height difference between will be given by:
[highest height-lowest height]
147-(-21)
=147+21
=168 m
the answer is 180 m