AB is parallel to DC (given).
AB is congruent to DC (given).
Both triangles share line AC...so you can start there and say that AC is congruent to AC (reflexive property).
Since AB and DC are parallel, and they are cut by a transversal (line AC), you can say that angle DCA is congruent to angle BAC (alternate interior angles theorem).
Since you have 2 sides and the included angle congruent on these 2 triangles, you can say that triangles ABC and CDA are congruent by the SAS (Side-Angle-Side) theorem...or just “SAS”
