6. (x-1) (2x + 1) + (x - 1) = (x + 1) (x - 1).

Решим уравнение:

1. $$(x - 1)(2x + 1) + (x - 1) = (x + 1)(x - 1)$$ $$2x^2 + x - 2x - 1 + x - 1 = x^2 - 1$$ $$2x^2 - 1 - 2 = x^2 - 1$$ $$2x^2 - x^2 = 2$$ $$x^2 = 2$$ $$x = \pm \sqrt{2}$$

Ответ: x = √2, x = -√2