If you know that two triangles are congruent, you know that any unmarked corresponding parts are also congruent by

The SSS, SAS, ASA, AAS, and HL theorems. These theorems state that if any of those combinations of angles or sides are congruent, the triangles are congruent. That means that you can apply it in reverse, because if two triangles are congruent then SSS, SAS, ASA, AAS, and HL (if a right triangle) are also all congruent.