Verified answer

(x² - 1)(x² + 3) = (x² + 1)² + x

x⁴ + 3x² - x² - 3 = x⁴ + 2x² + 1 + x

x⁴ + 2x² - 3 - x⁴ - 2x² - 1 - x = 0

- x - 4 = 0

- x = 4

x = - 4