Perhaps you doubled the 8 and 11 instead of squaring them. The sides are legs of a right triangle, and the diagonal is the hypotenuse. Use the Pythagorean Theorem: $\displaystyle{{8}}^{{2}}+{{11}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$ . Solve for c. The correct answer is $\displaystyle\sqrt{{185}}.\usepackage{cmbright}\Huge\$