\[y = \frac{64}{(x - 4)^{2} - (x + 4)^{2}} =\]
\[= \frac{64}{x^{2} - 8x + 16 - x^{2} - 8x - 16} =\]
\[= \frac{64}{- 16x} = - \frac{4}{x}\]