You need to prove that ΔAEC ≅ ΔBED
You know that point E is the midpoint of AB and CD
The midpoint divides a line segment into two congruent lines. This means that E divides AB into two congruent segments AE and EB, and it also divides CD into two congruent segments CE and ED, then we can conclude that:
1. AE=EB → Reason Midpoint Theorem
2. CE=ED → Reason Midpoint theorem
Both lines AB and CD intersect at point E, forming an X shape. The ingles formed inside the X are vertically opposite angles, this means that
3. ∠AEC= ∠BED → Reason Vertically opposite angles
Since E is the midpoint where both "diagonals of the parallelogram" AB and CD intersect, then sides AC and DB are parallel. If AB || CD, then ∠ACE and ∠BDE are alternate interiors angles as well as ∠CAE and ∠DBE. Alternate interior angles are congruent, so that:
4. ∠ACE=∠BDE → Reason alternate interior angles, AB || CD
5. ∠CAE=∠DBE → Reason alternate interior angles, AB || CD
Then → ΔAEC ≅ ΔBED by SAS and AAA