Question 141880
{{{64x^2-81}}}
This polynomial is of the form
{{{z^2-a^2}}}
which factors to 
{{{z^2-a^2=(z-a)(z+a)}}}
Let's look at each of the expressions.
{{{z^2=64x^2}}}
{{{z=8x}}}
{{{a^2=81}}}
{{{a=9}}}
So when you plug that in, you get,
{{{64x^2-81=(8x-9)(8x+9)}}}