In this video lesson, students practice simplifying complicated expressions in order to write an equation in a simpler form. Students learn that sometimes, it’s helpful to rewrite an expression in an equivalent form to understand how the quantities it represents are related. This work has some pitfalls when the expression has negative numbers or subtraction. This lesson is meant to inoculate students against common errors by reminding them that while subtraction is not commutative, addition is, and subtraction can be rewritten as adding the opposite. For example, 6x – 5 + 2x can be rewritten 6x + -5 + 2x and then rearranged 6x + 2x +-5. Likewise, 5x – 2(x + 3) can be rewritten 5x + -2(x + 3) before distributing -2.

Grade 7, Episode 18: Unit 6, Lesson 18 | Illustrative Math