salut :
6) 4x²+x=x(4x+1)
7) (x+1)(x+2)+5(x+1)=(x+1)[(x+2)+5]
=(x+1)(x+2+5)
=(x+1)(x+7)
8) (x+2)(x+3)-(x+2)(3x-1)=(x+2)[(x+3)-(3x-1)]
=(x+2)(x+3-3x+1)
=(x+2)(-2x+4)
9) (x+2)²+(x+2)(4x-5)=(x+2)[(x+2)+(4x-5)]
=(x+2)(x+2+4x-5)
=(x+2)(5x-3)
12) 9x²+24x+16=(3x)²+2(3x)(4)+4²=(3x+4)²
13) (x+1)² - 9 = (x+1)²-3²
=[(x+1)+3][(x+1)-3]
=(x+1+3)(x+1-3)
=(x+4)(x-2)