Chapter 5 - Section 5.4 - Multiplying Polynomials - Exercise Set - Page 289: 55

$25x^6-4y^2$

Using the product of the sum and difference of like terms, $(a+b)(a-b)=a^2-b^2$, the given expression is equivalent to \begin{align*} & (5x^3+2y)(5x^3-2y) \\\\&= (5x^3)^2-(2y)^2 \\\\&= 25x^6-4y^2 .\end{align*}