## Explanation

### Equivalent Expression to $6^{-3}$

To find an expression that is equivalent to $6^{-3}$, we need to understand the rules of exponents, particularly negative exponents.

**A negative exponent** indicates that the base should be taken as the reciprocal. Therefore:

Now, we can further simplify this expression by calculating $6^3$.

$6^3 = 6 \times 6 \times 6 = 216$Thus, the expression for $6^{-3}$ can be rewritten as:

$6^{-3} = \frac{1}{6^3} = \frac{1}{216}$Where:

- $6^{-3}$ is the original expression with a negative exponent.
- $\frac{1}{6^3}$ represents the reciprocal of $6^3$.
- $6^3$ equals $216$.

Therefore, the expression equivalent to $6^{-3}$ is:

$\boxed{\frac{1}{216}}$