20) $\sqrt{6 x}\left(2 x^3+\sqrt{12 x}\right)=$ $2x^3\sqrt{6x}+\sqrt{72x^2}$ $\boxed{2x^3\sqrt{6x}+6x\sqrt{2}}$ 22) $\frac{4 \sqrt[3]{8 x^4}}{\sqrt[3]{-20 x^3}}$ $\frac{4 \cdot2x\sqrt[3]{x}}{-x\sqrt[3]{20}}$ $\frac{8\sqrt[3]{x}}{-\sqrt[3]{20}}$ Don't forget to rationalize: $20=2^2\cdot5$, so we need $2\cdot5^2$ to make a perfect cube. $\frac{8\sqrt[3]{x}}{-\sqrt[3]{20}}\cdot\frac{\sqrt[3]{2\cdot5^2}}{\sqrt[3]{2\cdot5^2}}$ $\frac{8\sqrt[3]{50x}}{-10}=\boxed{\frac{4\sqrt[3]{50x}}{-5}}$ --- --- We simplify radicals because when two radical terms are simplified, their compatibility for addition/subtraction can be determined