because ac || de, the tiny triangle at the top shares the same angles as that of the large one.
because of this, the triangles sides are in ratio to each other.
knowing this, we see that the left side of the tiny triangle is 3, and the left side of the big triangle is 18.
In other words, the large triangle is 6 (18 / 3) times bigger than the small one.
we can apply this to the right side. if the small triangles right side is 4, we know the large triangles right side must be 6 times that, or 24.
if BC = 24, and we are looking for EC, we can subtract the length of BE (4) from 24 to get:
20