Correct. Given $\displaystyle{810}=\frac{{1}}{{3}}{\left({30}\right)}{{s}}^{{2}}={10}{{s}}^{{2}}\usepackage{cmbright}\Huge\$ , we know that $\displaystyle{{s}}^{{2}}=\frac{{810}}{{10}}={81}\usepackage{cmbright}\Huge\$ , so $\displaystyle{s}={9}\usepackage{cmbright}\Huge\$ .