When two figures are congruent, it means that they have the same shape and size. Therefore, their corresponding parts are also congruent. In this case, since Figure 1 and Figure 2 are congruent, their corresponding sides are also congruent.
According to the image you sent, side RP in Figure 1 corresponds to side IE in Figure 2.
I hope this helps! Let me know if you have any other questions.
