comment factoriser : 9(x+1)² -4x (2x+3)² -(x+1)(2x+3)

9(x^2+2x+1)-4x
9x^2+18x+9-4x
9x^2+14x+9

4x^2+12x+9 -2(x+1)(2x+3)
4x^2+12x+9 -(2x+2)(2x+3)
4x^2+12x+9 -2x(2-3)
4x^2+12x+9 -2x(-1)
4x^2+12x+9 -2x
4x^2+10x+9

9(x+1)² -4x
9(x+1)² = (3x+3)²
-4x = -(2√x)² 
(3x+3)² - (2√x)² 
(a²-b²) = (a+b)(a-b) (Identité remarquable)
(3x+3 + 2√x) (3x+3 -2√x)

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