Salut !
A = 8x - 56 = 8(x-7)
B = 7x²-21x = 7x(x-3)
C = 39x+26x² = 13x(3+2x)
D = (5x+4)(x-2)+(5x+4)(8-3x)
= (5x+4)[(x-2)+(8-3x)]
= (5x+4)(6-2x)
= 2(5x+4)(3-x)
E = (x+3)²+(x+3)(-2x+9)
= (x+3)(x+3)+(x+3)(-2x+9)
= (x+3)[(x+3)+(-2x+9)]
= (x+3)(12-x)
F = (x-7)²+(x-7)
= (x-7)(x-7)+(x-7)1
= (x-7)[(x-7+1]
= (x-7)(x-6)
