If you would like to know what is f(x), you can find this using the following steps:
f(x) = ?
2 * f(x) + f(1-x) = x^2
2 * f(x) = x^2 - f(1-x) /2
f(x) = (x^2 - f(1-x)) / 2
f(x) = x^2 / 2 - f(1-x) / 2
The correct result would be f(x) = x^2 / 2 - f(1-x) / 2.
