Description: Sal demonstrates how to solve the system of equations 9x + 3y = 15 and y - x = 5 using the substitution method. We're asked to solve and graph this system of equations here. And just as a bit of a review, solving a system of equations really just means figuring out the x and y value that will satisfy both of these equations. And one way to do it is to use one of the equations to solve for either the x or the y, and then substitute for that value in the other one.*Teacher Advisor is 100% free.