\[y = f(x);\ \ \ f(x) = x^{2}\]
\[f(x) = f(x + 5)\]
\[x^{2} = (x + 5)^{2}\]
\[x^{2} - (x + 5)^{2} = 0\]
\[(x - x - 5)(x + x + 5) = 0\]
\[- 5 \bullet (2x + 5) = 0\]
\[2x + 5 = 0\]
\[2x = - 5\]
\[x = - 2,5.\]
\[Ответ:\ - 2,5.\]