3) (x-3)(3x+1) - (2x+3)(4x-1);

3) (x-3)(3x+1) - (2x+3)(4x-1) = x(3x) + x(1) -3(3x) -3(1) - [2x(4x) + 2x(-1) + 3(4x) + 3(-1)] = 3x² + x - 9x - 3 - (8x² - 2x + 12x - 3) = 3x² - 8x - 3 - (8x² + 10x - 3) = 3x² - 8x - 3 - 8x² - 10x + 3 = -5x² - 18x