#### Answer

$-125i$

#### Work Step by Step

Using $i^2=-1$ and the laws of exponents, the given expression, $
(5i)^{3}
,$ is equivalent to
\begin{array}{l}\require{cancel}
5^3i^3
\\\\=
125i^3
\\\\=
125i^2\cdot i
\\\\=
125(-1)\cdot i
\\\\=
-125i
\end{array}