Supplementary angles definition: They add up to 180°
There are several ways to prove a parallelogram:
1. Opposite sides theorem converse
2. Opposite angles theorem converse
3. Parallelogram diagonals theorem converse
4. Parallel congruent sides theorem
∠P + ∠Q = 180° --1
∠P + ∠S = 180° --2
1: ∠P = 180° - ∠Q
Sub 1 into 2:
180° - ∠Q + ∠S = 180°
180° + ∠S = 180° + ∠Q
∠S = ∠Q
Or you can try saying the opposite sides are parallel, since they are interior angles and those are straight lines
