Find the coordinates of point Q in terms of a, b, and c

Also Since PQRS is a parallelogram, SQ and PR bisect each other. Use the coordinates to verify that SQ and PR bisect each other.

I tried to make it as neat as possible.

Part 1: Find the coordinates of point Q in terms of a, b, and c

Part 2: Use the coordinates to verify that SQ and PR bisect each other.