a – bok kwadratu
d=a\sqrt2 – przekątna kwadratu
d-a=6\\ a\sqrt2-a=6\\ a(\sqrt2-1)=6\\a=\frac{6}{\sqrt2-1}\\a=\frac{6}{\sqrt2-1}\cdot \frac{\sqrt2+1}{\sqrt2+1}\\ a=\frac{6(\sqrt2+1)}{2-1}\\ a=6(\sqrt2+1)
P=a^2=[6(\sqrt2+1)]^2=36(2+2\sqrt2+1)=36(3+2\sqrt2)[cm^2]