8. As shown in the right figure, a square piece of paper $A B C D$ is cut into two triangles along the diagonal $B D$. In the first operation, triangle $A B D$ is translated vertically downward by 3 cm to triangle $E F G$; in the second operation, triangle $E F G$ is translated vertically downward by another 5 cm to triangle $H J J$. The overlapping area of the two pieces of paper after the first operation is equal to the overlapping area of the two pieces of paper after the second operation. What is the area of the square piece of paper $A B C D$ in square centimeters? $\qquad$
$121$