Apply the exponent 3 to each factor individually: $\displaystyle\frac{{{{\left(-{2}\right)}}^{{3}}{{\left({{r}}^{{3}}\right)}}^{{3}}{{\left({{s}}^{{7}}\right)}}^{{3}}}}{{{{\left({r}\right)}}^{{3}}}}\usepackage{cmbright}\Huge\$

Simplify and use the power rule: $\displaystyle\frac{{-{8}{{r}}^{{{3}\cdot{3}}}{{s}}^{{{7}\cdot{3}}}}}{{{{r}}^{{{1}\cdot{3}}}}}=\frac{{-{8}{{r}}^{{9}}{{s}}^{{21}}}}{{{{r}}^{{3}}}}\usepackage{cmbright}\Huge\$

Use the quotient rule: $\displaystyle-{8}{{r}}^{{{9}-{3}}}{{s}}^{{21}}=-{8}{{r}}^{{6}}{{s}}^{{21}}\usepackage{cmbright}\Huge\$