You may have chosen this answer because $\displaystyle{63}={3}\cdot{21}\usepackage{cmbright}\Huge\$ . However, $\displaystyle\sqrt{{63}}=\sqrt{{{3}\cdot{21}}}=\sqrt{{3}}\cdot\sqrt{{21}}\usepackage{cmbright}\Huge\$ , using the rule $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{{a}}}\cdot\sqrt{{{b}}}\usepackage{cmbright}\Huge\$ . Because $\displaystyle\sqrt{{3}}\usepackage{cmbright}\Huge\$ is not equal to 3, $\displaystyle\sqrt{{{3}}}\cdot\sqrt{{{21}}}\ne{3}\sqrt{{21}}\usepackage{cmbright}\Huge\$ .