Explanation:
In order to meet at point F, line DF must travel from the top line to the bottom line. If it crosses through E, the midway, that means that it is halfway to the bottom line. This means the distance D E is equal to E F, because both are the distance from midpoint E to one of the sides (top or bottom).
If D E and E F are equal length, then AB and BF must be the length as well, as it is the length DF line takes to get halfway down from the top. Since they are the same length, A F must be twice the length of AB.
Hope this helped!