Notes
three rectangles as a triangle solution

Three Rectangles as a Triangle

Solution by Area of a Rectangle and Triangle

With the points labelled as above, $A$ is the midpoint of the short side of a rectangle and $C$ is the midpoint of the long side. Therefore, triangle $A B C$ has area one eighth of the area of the yellow rectangle. The overlapping regions comprise six of these triangles, so have area $3$. The total area is then $3 \times 4 - 3 = 9$.