What is the total length 2.2 metre up and 900 milimetres below?

Answer 1

Convert to the same units then the addition can be done:

1 mm = 1/1000 m

→ 2.2 m + 900 mm = 2.2 m + 900 × 1/1000 m = 2.2 m + 0.9 m = 3.1 m

Answer 2

It is 3.1 metres.