Chapter 4 - Polynomials - 4.6 Special Products - 4.6 Exercise Set - Page 277: 66

$x^2-8x^4+16x^6$

Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $ \left( x-4x^3 \right)^2 $, is equivalent to \begin{array}{l}\require{cancel} (x)^2+2(x)(-4x^3)+(-4x^3)^2 \\\\= x^2-8x^4+16x^6 .\end{array}