\[y = f(x);\ \ \ f(x) = x^{2}\]
\[f(x - 4) = f(x)\]
\[(x - 4)^{2} = x^{2}\]
\[(x - 4)^{2} - x² = 0\]
\[(x - 4 - x)(x - 4 + x) = 0\]
\[- 4 \bullet (2x - 4) = 0\]
\[2x - 4 = 0\]
\[2x = 4\]
\[x = 2\]
\[Ответ:\ 2.\]