It's not a rectangle. Look at the picture.
It is a parallelogram.
The area of the red rectangle:
[tex]A_{\boxed{ \ }}=(4+8)(6+3)=(12)(9)=108[/tex]
The areas of the right triangles:
[tex]A_1=\dfrac{1}{2}(3)(4)=6\\\\A_2=\dfrac{1}{2}(6)(8)=24[/tex]
The area of a parallelogram:
[tex]A=A_{\boxed{ \ }}-(2A_1+2A_2)\\\\A=108-(2\cdot6+2\cdot24)=108-(12+48)=108-60=48[/tex]
