Firstly, it is given that BA=DC and <BAC=<DCA. You can say that AC=CA through reflexive property. We now have two pairs of sides that are congruent and one pair of angles between them. Thus, we can use SAS to prove that the triangles are indeed congruent. Now, that the triangles are congruent BC=DA because of CPCTC (corresponding parts in congruent triangles are congruent)
