Correct. Two adjacent sides of the square form the legs of a right triangle ( $\displaystyle{a}={b}={3}\usepackage{cmbright}\Huge\$ ) where the diagonal of the square is the hypotenuse of the triangle. Letting the hypotenuse equal c, the Pythagorean Theorem says that $\displaystyle{{c}}^{{2}}={{a}}^{{2}}+{{b}}^{{2}}={{3}}^{{2}}+{{3}}^{{2}}={9}+{9}={18}\usepackage{cmbright}\Huge\$ , so $\displaystyle{c}=\sqrt{{{18}}}\usepackage{cmbright}\Huge\$ .