Chapter P - Preliminary Concepts - P.3 - Informal Geometry and Measurement - Exercises P.3 - Page 24: 30

Congruent

Since the two base angles of the triangle are identical, we see that the triangle is symmetrical about the seam where the paper is being folded. Thus, we know that the two triangles are identical. Corresponding sides of identical triangles are congruent, so we know that the two given sides will be congruent.