(1-2x-3x^2)+(-x^2+2x+1)
(3b+c^2-d)-(c^2-2d)

1) (1 - 2x - 3x²) + (- x² + 2x + 1) = 1 - 2x - 3x² - x² + 2x + 1 = - 4x² + 2

2) (3b + c² - d) - (c² - 2d) = 3b + c² - d - c² + 2d = 3b + d