(1) Completes a missing corner of this graph, that is, a square with a side length of 5cm, and the completed graph is a square with a side length of 5-3=2cm. Subtracting the area of a big square from the area of a complementary square is equal to the area of that figure.

5 x 5 - ( 5 - 3 ) x ( 5 - 3 ) = 2 1 (cm？ )

② Divide the graph into upper and lower parts. The upper rectangle is 3cm long, 5-3=2cm wide, and the lower rectangle is 5cm long and 3cm wide. The sum of the two parts is the area of the graph.

(5 - 3 ) x 3 + 5 x 3 = 2 1 (cm？ )