Suppose we add a small right triangle to the upper right corner of the whole graph, and it becomes a big rectangle with a length of 8+6= 14 cm and a width of 8 cm. (as shown in the figure below)

Then shadow area = large rectangle-upper left blank triangle-lower right blank triangle-newly added triangle.

The area of the big rectangle =(8+6)×8= 1 12 (square centimeter)

Area of the blank triangle in the upper left corner =8×8÷2=32 (square centimeter)

The area of the blank triangle in the lower right corner =(8+6)×6÷2=42 (square centimeter).

New triangle area =6×(8-6)÷2=6 (square centimeter)

Shadow area = 1 12-32-42-6=32 (square centimeter)