Let's solve the given problems step by step.

### Problem 64

#### a)
\[ \sqrt[3]{\frac{3}{4}} \times \sqrt[3]{\frac{9}{16}} \]

First, simplify each cube root:

\[ \sqrt[3]{\frac{3}{4}} \]
\[ \frac{3}{4} \text{ cannot be simplified further.} \]

\[ \sqrt[3]{\frac{9}{16}} \]
\[ \frac{9}{16} = \left( \frac{3}{4} \right)^2 \]
\[ \Rightarrow \sqrt[3]{\left( \frac{3}{4} \right)^2} = \left( \frac{3}{4} \right)^{2/3} \]

Multiply the cube roots:

\[ \sqrt[3]{\frac{3}{4}} \times \left( \frac{3}{4} \right)^{2/3} \]
\[ = \left( \frac{3}{4} \right)^{1/3} \times \left( \frac{3}{4} \right)^{2/3} \]
\[ = \left( \frac{3}{4} \right)^{(1/3 + 2/3)} \]
\[ = \left( \frac{3}{4} \right)^1 \]
\[ = \frac{3}{4} \]

So, the result is:

\[ \frac{3}{4} \]

#### b)
\[ \sqrt[3]{54} : \sqrt[3]{-2} \]

This represents division of the cube roots:

\[ \frac{\sqrt[3]{54}}{\sqrt[3]{-2}} \]

Combine the cube roots under a single fraction:

\[ \sqrt[3]{\frac{54}{-2}} \]
\[ \frac{54}{-2} = -27 \]
\[ \sqrt[3]{-27} = -3 \]

So, the result is:

\[ -3 \]