Yi De, For the rectangle at the bottom, (5x-y) x 2y, why did you end up with 10xy - 4y^2?Did you notice one part not quite right? The areas of the 2 other parts are correct. You have also not shown how to find the area of the required part by subtracting the various parts from the main square. Please key in this part in this SAME post.You have correctly shown how the expression can be factorised using distributive law. Please show how you could also do this using the FOIL method. You will eventually find the FOIL method giving you a systematic and quicker way of doing expansion.Insert the photo of this working in this SAME post, too.