## Intermediate Algebra (6th Edition)

$9x^{2a}-16$
Using $(a+b)(a-b)=a^2-b^2$ of the product of the difference of two squares, then, \begin{array}{l} (3x^a-4)(3x^a+4) \\= (3x^a)^2-(4)^2 \\= 9x^{2a}-16 .\end{array}