The Angle-Angle-Side Postulate (AAS) states that if two angles and a non-included side of one triangle are congruent to two angles and the corresponding non-included side of another triangle, then the two triangles are congruent.
Given that ∠A ≅ ∠J and AC ≅ JL, then we need that ∠B ≅ ∠K to satisfy postulate AAS (the two angles are ∠A and ∠B in one triangle and ∠K and ∠J in the other one, and the non-included side is AC in one triangle and JL in the other one).
