22.4
If the bottom side is 20 cm, then each line BE and CE would each be 10 cm.
Now, just focus on the top left triangle. You have one side with 20 cm, and the other side with 10 cm. You are left with the hypotenuse, which you need to figure out the value of.
Since this is a right triangle, you can use the Pythagorean theorem -> a^2 + b^2 = c^2 where c is the hypotenuse.
20^2 + 10^2 = c^2
400 + 100 = c^2
500 = c^2
22.36 = c
