Answer
$2^x=10$.
Work Step by Step
We know that $\frac{1}{1000}=\frac{1}{10^3}=10^{-3}$ and that $2^{-3x}=(2^x)^{-3}.$
Hence $2^{-3x}=\frac{1}{1000}\\(2^x)^{-3}=10^{-3}$.
We know that $a^b=c^b\longrightarrow a=c$ if $b\ne1$.
Thus $2^x=10$.