You may have rewritten the radical using a rational exponent, but you did not follow the laws of exponents to simplify. You applied the exponent $\displaystyle\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ only to the constant 16 and not to the other factors in the radicand. The proper way to start the simplification is to write the expression as: $\displaystyle{{16}}^{{\frac{{1}}{{4}}}}\cdot{{\left({{f}}^{{4}}\right)}}^{{\frac{{1}}{{4}}}}\cdot{{\left({{g}}^{{2}}\right)}}^{{\frac{{1}}{{4}}}}\cdot{{\left({{h}}^{{8}}\right)}}^{{\frac{{1}}{{4}}}}\usepackage{cmbright}\Huge\$