Chapter 5 - Section 5.4 - Multiplying Polynomials - Exercise Set - Page 289: 32

$49x^2-81$

Using the special product on multiplying the sum and difference of like terms, $(a+b)(a-b)=a^2-b^2$, the given expression simplifies to \begin{align*} & (7x-9)(7x+9) \\\\&= (7x)^2-9^2 \\\\&= 49x^2-81 .\end{align*}