Answer
$x(4+9x)(4-9x)$
Work Step by Step
Factoring the $GCF=
x
$, the given expression, $
16x-81x^3
,$ is equivalent to
\begin{array}{l}
x(16-81x^2)
.\end{array}
Using $x^2-y^2=(x+y)(x-y)$ or the factoring of the difference of 2 squares, then the factored form of the expression, $
x(16-81x^2)
,$ is
\begin{array}{l}
x(4+9x)(4-9x)
.\end{array}