How do you solve a simultaneous equation by 'substitution'?

Firstly, demonstrate with an example: Solve: 3x + y = -9 and x² + 2x - 3 = yRearrange the first equation to get "y" by itself by moving parts of the equation to the other side e.g. y = - 9 - 3xSubstitute your new value for "y" in terms of "x" into the second equation e.g. x² + 2x -3 = -9 - 3xMove all the terms onto the same side of the equation to make it equal 0 e.g. x² + 5x + 6 = 0Factorise your quadratic e.g. (x + 2)(x + 3) = 0 (you can check this by expanding the brackets back out using the 'FOIL' methodFrom this we know x = -2 and x = -3We then plug these values back into our equation we made earlier: y = -9 - 3xWhen x = -2, y = -3 and when x = -3, y = 0