This Instructional Activity is a part of the Lesson 21, Unit 3, Grade 7. They use polyhedron nets to understand that surface area is simply the sum of the area of the lateral faces and the area of the base(s). Students use prior knowledge to find the surface area of the given right rectangular prism by decomposing the prism into the plane figures that represent its individual faces. The net represents all the surfaces of the rectangular prism, so its area is equal to the surface area of the prism.