Which one of the following is not a quadratic equation
a. (x + 2)² = 2(x + 3)
b. x² + 3x = (-1) (1 - 3x)²
c. (x + 2) (x - 1) = x² - 2x - 3
d. x³ - x² + 2x + 1 = (x + 1)³