You have to FOIL out the (x+4)(x-4) and then subrtract away the 9 in order to get a quadratic that you can solve for x. As it is, you can't do it.
[tex](x+4)(x-4)= x^{2} -4x+4x-16= x^{2} -16[/tex]
Now if you move the 9 over with it, you get this:
[tex] x^{2} -16-9=0[/tex]
which simplifies to
[tex] x^{2} -25=0[/tex]
Now you can either solve this by recognizing that is the difference of perfect squares, or you can move the 25 over to the other side and take the square root of both sides, like this:
[tex] x^{2} =25[/tex]
[tex] \sqrt{ x^{2} } = \sqrt{25} [/tex]
[tex]x=+/-5[/tex]
