You may have used the Pythagorean Theorem with 8 as a leg and the 11 as the hypotenuse. However, the diagonal is the hypotenuse. This gives you the equation $\displaystyle{{8}}^{{2}}+{{11}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$ . Solve for c. The correct answer is $\displaystyle\sqrt{{185}}.\usepackage{cmbright}\Huge\$