The purpose of this lesson is to practice combining like terms and finding vertical and horizontal lines that pass through the point representing the solution of a system of equations. In the associated Algebra 1 lesson, students begin the process of understanding how to solve systems of equations by elimination. When adding or subtracting equations, students need to combine like terms. The first activity after the Warm-up gives students a chance to practice that skill. To understand solving by elimination, students in the associated Algebra 1 lesson look at graphs representing various equations that result from adding two equations where one variable is eliminated. In particular, the resulting equations are of the form  or , for a number . Recalling the appearance of graphs that represent such equations is supported by the second activity in this lesson. Students reason abstractly and quantitatively (MP2) when creating and working with equivalent equations.