Question 1205225
One should, if too hard to visualize ,  draw the description and develop the area formula from that.


x for the height
2x for the width
of the whole sign piece


Each corner  is {{{1*(1/3)}}} ft*ft and there are four of them so all the rectangular corners make  {{{4/3}}} square feet.
(I chose units of feet; not inches.)


The sign's area limited to {{{3600*in^2*(1/12)(1/12)(ft/in)^2=3600/144=25*ft^2}}}.
{{{3600(1/12)(1/12)=3600/144=25}}}{{{squarefeet}}}


The whole sign, borders plus printable area
{{{2x*x=25}}}

{{{x^2=25/2}}}

{{{x=5/sqrt(2)}}}---------you can rationalize if desired.


You can figure out what to do about finding the printable region part.
This is not so much about the corners but the ENTIRE border region.