You recognized that you can rewrite $\displaystyle{63}={9}\cdot{7}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle\sqrt{{63}}=\sqrt{{{9}\cdot{7}}}\usepackage{cmbright}\Huge\$ . However, this result is not equal to $\displaystyle{9}\sqrt{{{7}}}\usepackage{cmbright}\Huge\$ . You need to use $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ to rewrite the expression as $\displaystyle\sqrt{{9}}\cdot\sqrt{{7}}\usepackage{cmbright}\Huge\$ , and then simplify further.