Correct. By the Pythagorean Theorem, if c is the length of the diagonal, then $\displaystyle{{c}}^{{2}}={{5}}^{{2}}+{{12}}^{{2}}={25}+{144}={169}\usepackage{cmbright}\Huge\$ , so $\displaystyle\sqrt{{{{c}}^{{2}}}}=\sqrt{{{169}}}={13}\usepackage{cmbright}\Huge\$ , and therefore c = 13.