ab = x + 15 ad = 2x + 18 bc = 3 - x cd = 3x + 25 quadrilateral abcd is a parallelogram if both pairs of opposite sides are congruent. prove that quadrilateral abcd is a parallelogram by finding the value of x. a) x = 3 b) x = -5 c) x = -11 d) x = 11/2