Answer
$\frac{1}{x^{18}}$
Work Step by Step
According to the power rule for exponents, $(a^{m})^{n}=a^{mn}$ (where $m$ and $n$ are integers and $a$ is a real number).
Therefore, $(x^{-3})^{6}=x^{-3\times6}=x^{-18}$.
According to the negative definition for exponents, $a^{-n}=\frac{1}{a^{n}}$ (where $a\ne0$).
Therefore, $x^{-18}=\frac{1}{x^{18}}$.