In a quadrilateral PQRS, opposite angles are equal.

Since, the opposite angles of the quadrilateral PQRS are equal.

⇒">⇒ Quadrilateral PQRS is a parallelogram.

$\Rightarrow S R=P Q$ (Opposite sides of a parallelogram are equal.)

$\therefore S R=P Q=2 \mathrm{~cm}$