This is an example of a factor pattern called the Difference of Two Squares.
[tex]A^2-B^2=(A+B)(A-B)[/tex]
What is playing the role of A? What do you square to get [tex]9x^2[/tex]? That would be [tex]3x[/tex].
What is playing the role of B? What do you square to get 64? That would be 8.
Your expression factors as [tex](3x+8)(3x-8)[/tex]
