The coordinates of quadrilateral pqrs are p(0, 0), q(a+c,0), r(2a +c, b), and s(a, b). how can you use coordinate geometry to show that one pair opposite sides is parallel? find the slopes of pr and qs, and show that their product is -1. find the slopes of pr and qs, and show that their product are equal. find the slopes of pq and rs, and show that their product is 1. find the slopes of pq and rs, and show that they are equal.