Learning goals: Teacher Facing Recognize that a system can be efficiently solved by substitution if one variable is already isolated or can be easily isolated. Recognize that there are multiple ways to perform substitution to solve a system of equations. Solve systems of linear equations by substituting a variable with a number or an expression, and check solutions by substituting them back into the equations. Full lesson plan (warm-up, activities, synthesis, cool-down prompt) from Alg1.2 Linear Equations, Inequalities, and Systems, Algebra 1.