2012 AMC 12B Question 17

Square $PQRS$ lies in the first quadrant. Points $(3, 0),$ $(5, 0),$ $(7, 0),$ and $(13, 0)$ lie on lines $SP,$ $RQ,$ $PQ,$ and $SR,$ respectively. What is the sum of the coordinates of the center of the square $PQRS?$