This Instructional Activity is a part of the Lesson 15, Unit 6, Grade 7. Students use conditions that determine a unique triangle to construct viable arguments that angle measures and lengths are equal between triangles. In Lesson 15, students continue to apply their understanding of the conditions that determine a unique triangle. Lesson 15 exposes students to yet another challenge where they are asked to determine whether triangles are identical and then to show how this information can lead to further conclusions about the diagram (i.e., showing why a given point must be the midpoint of a segment).