6. As shown in the right figure, $A F=7 \text{ cm}, D H=4 \text{ cm}, B G=5 \text{ cm}, A E=1 \text{ cm}$. If the area of quadrilateral $E F G H$ inside the square $A B C D$ is $78 \text{ cm}^2$, then the side length of the square is $(\quad \text{cm}$.
A. 10
B. 11
C. 12
D. 13
【Answer】C
【Analysis】 $I L=7-4 \mathrm{~cm}, L K=5-1=4 \mathrm{~cm}$, so $S_{I L K J}=12 \mathrm{~cm}$, $(78-12) \times 2=144 \mathrm{~cm}^{2}$, so the side length is $12 \mathrm{~cm}$. Choose C.