If two angles of a triangle are congruent, then the sides opposite to these angles are congruent.

Proof

Draw $\overline{SR}$, the bisector of the vertex angle $\angle PRQ$.

Since $\overline{SR}$ is the angle bisector, $\angle PRS\cong \angle QRS$.

By the Reflexive Property,

$\overline{RS}\cong \overline{RS}$

It is given that $\angle P\cong \angle Q$.

Therefore, by AAS congruent, $\Delta PRS\cong \Delta QRS$.

Since corresponding parts of congruent triangles are congruent,

$\overline{PR}\cong \overline{QR}$