Question 1189612
ONLY if this piece of wood is very flat, and you are only interested in something like two dimensions:


Original rectangle's area,  {{{12*6=72}}} square inches


An area of {{{4*x^2}}} removed from all the four corners.


Remaining area,  20 square inches.

{{{highlight_green(72-4x^2=20)}}}

{{{72-20=4x^2}}}

{{{52=4x^2}}}

{{{13=x^2}}}

{{{highlight(x=sqrt(13))}}}