Correct. Two adjacent sides of the rectangle, along with the diagonal, forms a right triangle. Using $\displaystyle{a}={6}\usepackage{cmbright}\Huge\$ and $\displaystyle{c}={9}\usepackage{cmbright}\Huge\$ , the Pythagorean Theorem says that $\displaystyle{{a}}^{{2}}+{{b}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{{6}}^{{2}}+{{b}}^{{2}}={{9}}^{{2}}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle{{b}}^{{2}}={{9}}^{{2}}-{{6}}^{{2}}={81}-{36}={45}\usepackage{cmbright}\Huge\$ , so $\displaystyle{b}=\sqrt{{{45}}}\usepackage{cmbright}\Huge\$ .