The substitution method is a way to solve a system of two equations by replacing one variable with an equivalent expression. When you can solve one of the equations for a variable, you can substitute that expression into the other equation to find the solution. For this system: −4x y = 6 −5x − y = 21 Step 1: Solve one equation for a variable. The first equation is easy to rearrange. Add 4x to both sides: y = 6 4x Step 2: Substitute that expression into the second equation. Replace y in the secon