Use the laws of exponents to simplify the expression:

$\displaystyle{{\left({256}{{x}}^{{16}}{{y}}^{{2}}\right)}}^{{\frac{{1}}{{4}}}}={{256}}^{{\frac{{1}}{{4}}}}\cdot{{x}}^{{\frac{{16}}{{4}}}}\cdot{{y}}^{{\frac{{2}}{{4}}}}={{\left({4}\cdot{4}\cdot{4}\cdot{4}\right)}}^{{\frac{{1}}{{4}}}}\cdot{{x}}^{{4}}\cdot{{y}}^{{\frac{{1}}{{2}}}}\usepackage{cmbright}\Huge\$

$\displaystyle={{\left({{4}}^{{4}}\right)}}^{{\frac{{1}}{{4}}}}{{x}}^{{4}}{{y}}^{{\frac{{1}}{{2}}}}={{4}}^{{\frac{{4}}{{4}}}}{{x}}^{{4}}{{y}}^{{\frac{{1}}{{2}}}}={4}{{x}}^{{4}}{{y}}^{{\frac{{1}}{{2}}}}={4}{{x}}^{{4}}\sqrt{{y}}\usepackage{cmbright}\Huge\$