From rotation angle AFD= angle AF'E, ∠F'AB=∠DAF, DF=BF'

Because square ABCD

So the angle D = 90 and the angle BAD = 90.

So the angle DAF+AFD = 90°, that is, ∠ AF 'b+∠ DAF = 90, 2∠F'AB (because ∠ DAF = ∠ EAF = ∠ F 'AB)+BAE = 90.

So ∠AF'B=∠F'AE.

So AE=F'E

Because F'E=F'B+BE=FD+BE.

So AE=FD+BE

If you still can't, you can ask me. I hope my answer will help you.