You may have correctly rewritten the radical as $\displaystyle{{\left({36}{{a}}^{{2}}{{b}}^{{8}}\right)}}^{{\frac{{1}}{{4}}}}\usepackage{cmbright}\Huge\$ , but you did not use the laws of exponents to simplify. You used 4 as the numerator and each factor's exponent as a denominator before simplifying. You should have multiplied each factor's exponent by $\displaystyle\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ instead.