Diagonal?

The diagonal of a square is given by $d=x\sqrt{2}$ where $x$ is the side length. The side length of the square in the problem is $\dfrac{12}{4}=3$ . So the length of the diagonal is $\boxed{3\sqrt{2}}$ .

The side length of the square is $\dfrac{12}{4}=3$ . Let $x$ be the diagonal. By pythagorean theorem, we have

$x^2=3^2+3^2$

$x^2 = 9 +9$

$x^2 = 18$

$x=\sqrt{18}=\sqrt{9*2}=\sqrt{3^2*2}=\boxed{3\sqrt{2}}$

Consider my diagram. If $d$ is the diagonal and $a$ is the side length of a square, then $d=a\sqrt{2}$ . The perimeter is $12$ , so the length of each side must be $\dfrac{12}{4}=3$ . Hence, $d=3\sqrt{2}$ .

$P=4s$

$12=4s$

$s=3~cm$

By pythagorean theorem,

$d=\sqrt{3^2+3^2}=\sqrt{9(2)}=3\sqrt{2}$ $\boxed{\text{answer}}$