(sqrt3(-(8*a^3))/b^21)
This might mean the cube root of -8a3/b21 and the brackets are in the wrong place. If so, then
$$\sqrt[3]{\frac{-8a^3}{b^{21}}}\\\\ \frac{\sqrt[3]{-8}\times \sqrt[3]{a^3}}{\sqrt[3]{b^{21}}}\\\\ \frac{-2a}{b^7}$$
.
$$\frac{\sqrt3(-(8*a^3))}{b^{21}}\\\\ =\frac{\sqrt3(-8*a^3)}{b^{21}}\\\\ =\frac{-8a^3\sqrt3}{b^{21}}\\\\$$
That is all you can do with this.