Appendix A - Review - A.7 nth Roots; Rational Exponents - A.7 Assess Your Understanding - Page A60: 13

$-2x\sqrt[3] x$

For positive real numbers $a$ and $b$, $\sqrt[n]{a}\cdot \sqrt[n]{b}=\sqrt[n]{ab}$. Thus, $ \sqrt[3]{-8x^4}=\sqrt[3]{(-8x^3)\cdot x}=\sqrt[3]{-8x^3}\cdot\sqrt[3]{x}=\sqrt[3]{(-2x)^3} \cdot \sqrt[3]{x}=-2x\sqrt[3]{x} $