We have
[tex]16=2^4\implies\sqrt[4]{16}=2[/tex]
[tex]81=3^4\implies\sqrt[4]{81}=3[/tex]
[tex]32=2^5\implies\sqrt[4]{32}=2\sqrt[4]{2}[/tex]
So
[tex]2\sqrt[4]{16x}-2\sqrt[4]{2y}+3\sqrt[4]{81x}-4\sqrt[4]{32y}[/tex]
is equivalent to
[tex]2^2\sqrt[4]{x}-2\sqrt[4]{2y}+3^2\sqrt[4]{x}-8\sqrt[4]{2y}[/tex]
which reduces to
[tex]13\sqrt[4]{x}-10\sqrt[4]{2y}[/tex]