Lesson objective: Solve systems of equations by substituting equivalent expressions for a variable to write and solve an equation for the other variable.

This lesson helps to build procedural skill with substituting equivalent expressions. Equations are used here because they support multiple representations of the same quantity in different ways. This work develops students' understanding that systems of equations can be solved by applying properties of equality to write and solve equations.

Students engage in Mathematical Practice 1 (make sense of problems and persevere in solving them) as they make sense of equivalent expressions representing the same quantity and then use the equivalent to write and solve equations.

**Key vocabulary:**

- equivalent expression
- properties of equality
- properties of operation
- substitution