Correct. The sides of the rectangle are the legs of a right triangle, and the diagonal of the rectangle is the hypotenuse of the triangle. That is, $\displaystyle{a}={3}\usepackage{cmbright}\Huge\$ , $\displaystyle{b}={5}\usepackage{cmbright}\Huge\$ , and c is the length of the diagonal. Using the Pythagorean Theorem, $\displaystyle{{c}}^{{2}}={{a}}^{{2}}+{{b}}^{{2}}\usepackage{cmbright}\Huge\$ , you found that $\displaystyle{c}=\sqrt{{{34}}}\usepackage{cmbright}\Huge\$ , which is approximately 5.83.