X = 12
AB = X | BE = 5 | EC = 5 | FE = 6 | FG = 4
FE and AB are parallel lines. FE appears to be half of AB.
We know that BE and EC are half of BE.
BC = BE + EC.
5 + 5 = 10
Or
EC x 2 = BC
5 x 2 = 10
AB is half of FE (6 x 2) and BC is half of EC (5 x 2)
To find X we must multiply FE by 2 like we multiplied EC by 2.
X = 6 x 2
X = 12
Look at the picture for another explanation. I found it as I was looking for proof of my answer.