Answer:
you can easily forgot the lessob because of quarantine
ED:8cm,DC:10 so the air is HD:18
Use the geometric mean for right triangles here. Like this

. Cross multiply to get

, and

. That simplifies down to

. Pull out the 9 as a perfect square of 3 and you're left with

, first choice above.
I believe the answer is either C or D... I am not sure though..