To simplify the expression, find the cube root of 729 and the cube root of $\displaystyle{{s}}^{{9}}\usepackage{cmbright}\Huge\$ .

$\displaystyle{\sqrt[{{3}}]{{{729}{{s}}^{{9}}}}}={\sqrt[{{3}}]{{{9}\cdot{9}\cdot{9}\cdot{{\left({{s}}^{{3}}\right)}}^{{3}}}}}={\sqrt[{{3}}]{{{{9}}^{{3}}\cdot{{\left({{s}}^{{3}}\right)}}^{{3}}}}}={\sqrt[{{3}}]{{{{9}}^{{3}}}}}\cdot{\sqrt[{{3}}]{{{{\left({{s}}^{{3}}\right)}}^{{3}}}}}={9}{{s}}^{{3}}\usepackage{cmbright}\Huge\$