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[x(x² - 2x + 4)]-[3(x² - 2x + 4)]

x³ - 2x² + 4x - (3x² - 6x +12)

x³ - 2x² + 4x - 3x² + 6x - 12

x³ - 5x² + 10x -12

Multiplying the x through the second polynomial

x* x^2 = x^3

x * (-2x) = -2x^2

x * 4 = 4x

Multiplying the -3 through the second polynomial

-3*x^2 = -3x^2

-3 * (-2x) = 6x

-3 * 4 = -12

-2x^2 - 3x^2 = -5x^2

4x + 6x = 10x

The final answer is

x^3 - 5x^2 + 10x - 12

x^3 - 5x^2 + 10x - 12

(x-3)(x^2-2x+4)=

x^3-2x^2+4x-3x^2

+6x-12=

x^3-5x^2+10x-12

......x^2 - 2x + 4

...............x - 3

x^3 - 2x^2 + 4x

.....-3x^2 + 6x - 12

x^3 - 5x^2 + 10x - 12