\[\sqrt[4]{\text{ab}}\ :\sqrt[4]{b} + 2\sqrt{\sqrt{a}} =\]
\[= \sqrt[4]{\frac{\text{ab}}{b}} + 2\sqrt[4]{a} =\]
\[= \sqrt[4]{a} + 2\sqrt[4]{a} = 3\sqrt[4]{a}.\]