You may have correctly rewritten the expression as $\displaystyle\frac{{{{s}}^{{3}}{{\left({36}{{r}}^{{9}}{{s}}^{{4}}\right)}}^{{\frac{{1}}{{2}}}}}}{{{4}{s}{{\left({27}{{r}}^{{12}}\right)}}^{{\frac{{1}}{{3}}}}}}\usepackage{cmbright}\Huge\$ , but then you applied the rules of exponents to expressions within the parentheses and ignored the terms outside the parentheses. Apply the rule of exponents to multiply by terms outside the parentheses before simplifying further.