The purple one is the original square, and the side length is increased by 2 cm. As can be seen from the figure, the added parts are two rectangles and a square. The area of a square is 2*2=4, so the area of two rectangles is 20-4= 16, and the area of one rectangle is 8. The area of the rectangle =2* the side length of the original square, so the side length of the original square =8 divided by 2=4 cm, hope to adopt!