Question 827563
Nice general question.


Just translate the worded description into symbols and form the area calculation.  Simplify.


Sides, a and b.
Remove x by x square from each corner.


{{{highlight((ab)-4x^2=A)}}}----
That's it!  The area before removing any x is just {{{a*b}}}.  
Each square at the corner is {{{x^2}}}, and the original rectangle piece has four of them; so removing those is {{{-4x^2}}}.