p(x) = x * (12 - x) / (x - 6)

тогда

p(12-x) = (12 - x) * x / (6 - x) = - x * (12 - x) / (x - 6) = -p(x)

тогда

p(x) + p(12-x) = p(x) - p(x) = 0