The equation for area of a circle is a= pie r^2. Now plug in pie as 3.14 and you radius which would be 3 because the image is showing the diameter as 6 and radius is half the diameter. The equation is now a=3.14 x3x3. The area of the circle would be 28.27cm^2.
Now we must find the area of the square. To do this multiple the diameter which is 6 times itself. 6x6=36. The area of the square is 36cm^2.
Now subtract the square area from the circle area. 36 - 28.27 = 7.73. The left over plywood would be 7.73cm^2.
I hope this helps! If you have anymore questions feel free to ask! :D
