The reasons should be as follows;
1. Given
2. All right angles are congruent.
3. If two lines are parallel, then alternate interior angles are congruent.
4. By the ASA (Angle-Side-Angle) Postulate, the triangles are congruent.
What tells us about the triangles' sides being congruent?
Given that triangles △ABO and △DEO are right triangles, segment AB is congruent to segment DE, and line AB is parallel to line DE.
∠B ≅ ∠E Because of the right angles, as seen in the diagram.
∠A ≅ ∠D This is justified by the fact that if AB∥DE and AD is a transversal, then ∠A and ∠D are alternate interior angles, which are congruent.)
The Angle-Side-Angle (ASA) Postulate states that if two angles and a non-included side in one triangle are congruent to the corresponding angles and the corresponding side in another triangle, then the two triangles are congruent.