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)