Simplify the following expressions as much as possible. \text { a. }\left(2 x^{2} y^{3}\right)^{2}\left(-3 x y^{4}\right)^{3} 8 \sqrt[3]{2} \times \frac{1}{2} \sqrt[3]{16} \times \frac{3}{4} \sqrt[3]{2} \frac{4 \sqrt{b}}{\sqrt{3}-b}