Answer
$-y\sqrt[3]{3x}$
Work Step by Step
$$\sqrt[3]{24xy^3} - y\sqrt[3]{81x}$$ $$=\sqrt[3]{8y^3\times3x} - y\sqrt[3]{27\times3x}$$ $$=2y\sqrt[3]{3x} - 3y\sqrt[3]{3x}$$ $$=(2y-3y)\sqrt[3]{3x}$$ $$=-y\sqrt[3]{3x}$$
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