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2 shaded triangle → 81 cm²
2 same triangle, with equal base and height, can form a square
square area → 81 cm²
since square has same sides
side x side → 81 cm²
9cm x 9cm = 81 cm²
1 side of the square is 9cm, which also means xy is 9cm length of rectangular piece of paper → 20 + 9 + 12 + 9 + 20 = 70
area of rectangular piece of paper → 70 x 9 = 630 cm²