It is a parallelgram that is cut in half by a diagnoal line making two triangles
Here is another approach. After drawing the parallelogram try to prove that the two triangles formed by drawing the diagonal are congruent. Since we have a parallelogram we already know that AB=CD and that BC=DA. Since the diagonal BD is the same line for both triangles then we can prove congruency by SIDE SIDE SIDE. Congruent triangles have equal perimeters.