Answer
$\dfrac{1}{25}-36x^8$
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, $
\left( \dfrac{1}{5}-6x^4 \right)\left( \dfrac{1}{5}+6x^4 \right)
$, is equivalent to
\begin{array}{l}\require{cancel}
\left( \dfrac{1}{5} \right)^2-(6x^4)^2
\\\\=
\dfrac{1}{25}-36x^8
.\end{array}
