This lesson is optional because it includes opportunities to practice identifying and writing equivalent expressions that not all classes may need. Students look for structure in expressions to match equivalent expressions based on the distributive property (MP7). They also use the distributive property to generate equivalent sums or differences when given a product, and to generate an equivalent product when given a sum or difference (such as ). The latter includes opportunities for students to look for a common factor that is not the variable (such as in ).