Parallelogram abcd contains two congruent triangles. in two or more complete sentences, use the triangles to prove opposite sides are congruent in parallelogram abcd.

Ok so if I am imagining what I think the diagram is correctly, you could say that because they are congruent triangles their sides will be congruent. If their sides are congruent and the lines that you are trying to prove are part of the triangle, they will be congruent due to the CPCTC theorem. The CPCTC theorem just says that if two triangles are congruent, their corresponding parts are congruent.