Answer
$75x^3-147x$
Work Step by Step
Using $(a-b)(a+b)=a^2-b^2$, or the product of the sum and difference of like terms, the given expression, $
3x(5x+7)(5x-7)
,$ is equivalent to
\begin{array}{l}\require{cancel}
3x[(5x)^2-(7)^2]
\\\\=
3x(25x^2-49)
.\end{array}
Using the Distributive Property, the expression, $
3x(25x^2-49)
,$ is equivalent to
\begin{array}{l}\require{cancel}
75x^3-147x
.\end{array}