Chapter 5 - Exponents and Polynomials - 5.3 - Multiplying Polynomials - Problem Set 5.3 - Page 208: 108

$49x^2-81y^2$

Using $(a-b)(a+b)=a^2-b^2$, or the product of the sum and difference of like terms, the given expression, $ (7x-9y)(7x+9y) ,$ is equivalent to \begin{array}{l}\require{cancel} (7x)^2-(9y)^2 \\\\= 49x^2-81y^2 .\end{array}