A diagonal of a square is defined, [tex]d=a\sqrt{2}[/tex] where a is side of the square. Solve for a to get [tex]a=\frac{d}{\sqrt{2}}[/tex].
Now multiply a by 4 since that is the definition of a perimeter,
[tex]P=4a=4\cdot\frac{18\sqrt{2}}{\sqrt{2}}=4\cdot18=\boxed{72}[/tex].
Hope this helps.
