Answer:
- alternate interior angles are congruent
- corresponding parts of congruent triangles are congruent
- definition of a parallelogram
Step-by-step explanation:
Step Statement                          ⇒  Reason
1   RSTU is a parallelogram                ⇒  Given
2  Side RS || Side TU and Side RU || Side TS  ⇒ definition of a parallelogram
3   ∠RSU≅∠TUS and ∠RUS≅∠TSU         ⇒ alternate interior angles are congruent
4   Side SU ≅ Side US                    ⇒ reflexive property
5   ΔRSU≅ΔTUS                         ⇒ ASA postulate
6   Side RS ≅ Side TU and Side RU ≅ Side TS ⇒ corresponding parts of congruent triangles are congruent