Answer
$\dfrac{1}{27}$
Work Step by Step
In order to solve the given problem, we will use the following two rules:
$ (a) a^{-p}=\dfrac{1}{a^p} \\ (b) a^{pq}=(a^p)^q$
We can re-write as: $5^{3x}=(5^{x})^{3}$
or, $5^{3x}=(5^{-x})^{-3}$
Since, $5^{-x}=3$, then we simplify the expression as: $5^{3x}=(5^{-x})^{-3}$
Now, we will use Rule-(a) as: $5^{3x}=\dfrac{1}{3^3}$
Therefore, $5^{3x}=\dfrac{1}{27}$
