Answer
$36x^8-36x^5+9x^2$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
\left( 6x^4-3x \right)^2
$, is equivalent to
\begin{array}{l}\require{cancel}
(6x^4)^2+2(6x^4)(-3x)+(-3x)^2
\\\\=
36x^8-36x^5+9x^2
.\end{array}