In this lesson, students practice applying the Pythagorean Theorem to find unknown side lengths of right triangles. They begin by examining four equations of the type they will encounter later in the lesson, noting any similarities and differences. Students continue by studying a series of triangles and determining which side of each right triangle is the hypotenuse, reinforcing the idea that only right triangles have a hypotenuse and refining students' precision of language (MP6). In the last activity, students practice finding the unknown leg or hypotenuse given drawings or a written description.