AD=CE

A quadrilateral is a parallelogram.

∴AC∥DE

AC=DE

∵AC⊥BD

∴ Angle bde = 90

∵ isosceles trapezoid ABCD

∴AB=CD

∴BD=DE

∴△BDE is an isosceles triangle

CE = AD = 2

BC=4

∴BE=6

According to the trinity

DF=3

If you don't understand, please ask.