In the diagram, we have that CE and ED are congruent.
We also know that ∠FEC≅∠FED; this is because these two angles form a linear pair. A linear pair has a sum of 180°; since m∠FED = 90°, this means that m∠FEC = 180-90 = 90°, and the two angles are congruent.
We also know that FE = FE, due to the reflexive property.
This gives us two sides and the angle between them, or SAS.