\[(3x - 9)^{2} - (3x + 3)^{2} = 0\]
\[(3x - 9 - 3x - 3)(3x - 9 + 3x + 3) = 0\]
\[- 12 \cdot (6x - 6) = 0\]
\[- 12 \cdot 6(x - 1) = 0\]
\[x = 1.\]