The scrapbook page is a rectangle. The diagonal and the sides form a right triangle. Use the Pythagorean Theorem:

$\displaystyle{{8}}^{{2}}+{{11}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{64}+{121}={{c}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{185}={{c}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle\sqrt{{185}}={c}\usepackage{cmbright}\Huge\$