You correctly recognized that you can rewrite $\displaystyle{48}={16}\cdot{3}\usepackage{cmbright}\Huge\$ . Therefore, $\displaystyle\sqrt{{48}}=\sqrt{{{16}\cdot{3}}}\usepackage{cmbright}\Huge\$ . However, this is not equal to $\displaystyle{16}\sqrt{{{3}}}\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{{16}}\cdot\sqrt{{3}}\usepackage{cmbright}\Huge\$ , and then simplify further.