Question 81613
you can prove that the 2 triangles formed by drawing the diagonal are congruent by SSS ... you are given that the opposite sides are congruent and the diagonal is congruent to itself


using the congruency of the two triangles, you can prove that the alternate angles at the ends of the diagonal are congruent (corresponding parts of congruent triangles are congruent) ... this proves the parallelism of the opposite sides by equal alternate interior angles