The figure is not drawn to scale. It is made up of a square and a rectangle. The ratio of the area of the square to that of the rectangle is 3 : 4. After the shaded part is cut out, the ratio of the area of unshaded part of the rectangle to that of the unshaded part of the square is 2 : 1. Given the length of the square is 9 cm, find the area of the shaded part.