Question

Hello!

Answer 1

Let O be the point of intersection of diagonals.

Consider triangles FOE and EOD:
FO = OD,  ∠FOE = ∠EOD  and OE is a common side  ⇒ 
triangles FOE and EOD are congruent.

In congruent triangles all corresponding sides and are congruent  ⇒
EF = DE = 6