Correct. By the Pythagorean Theorem, if b is the length of the rectangle, then $\displaystyle{{20}}^{{2}}={{8}}^{{2}}+{{b}}^{{2}}\usepackage{cmbright}\Huge\$ , so $\displaystyle{{b}}^{{2}}={400}-{64}={336}\usepackage{cmbright}\Huge\$ . Since $\displaystyle\sqrt{{{{b}}^{{2}}}}=\sqrt{{{336}}}=\sqrt{{{16}\cdot{21}}},{b}={4}\sqrt{{{21}}}\usepackage{cmbright}\Huge\$ .