33. There are two types of square grass mats, as shown in Figure 10. The side length of the larger mat is 3 cm longer than that of the smaller mat, and the area of the larger mat is 63 square cm larger than that of the smaller mat. There is a square open space that can be fully covered by these two types of grass mats (the mats cannot be cut), and it can be fully covered by both the larger and smaller mats. The minimum area of this open space is $\qquad$ square cm.
Reference answer: 1296