We are given the following triangle:
We are given that:

Since RS is a bisector this means that:

Also, since S is a midpoint of PQ:

By the reflexive property we have:

Since we have two pairs of congruent angles this means that:

Therefore, by SAS theorem of congruency:

Since the triangles are congruent this means that all of its sides are congruent too, therefore:

And thus we have concluded the proof.