Bonjour !
[tex]A = 3x(x + 1) + 2x = \\ = 3 {x}^{2} + 3x + 2x \\ = 3 {x}^{2} + 5x[/tex]
[tex]B = (x+7)(x-2) \\ = {x}^{2} - 2x + 7x - 14 \\ = {x}^{2} + 5x - 14[/tex]
[tex]C = (5x+6) (2x - 1) \\ = 10 {x}^{2} - 5x + 12x - 6 \\ = 10 {x}^{2} + 7x - 6[/tex]
[tex]D = (x+2)(x - 5) + (x - 3)(x + 4) \\ = {x}^{2} - 5x + 2x - 10 + {x}^{2} + 4x - 3x - 12 \\ = {2x}^{2} - 2x - 22[/tex]
[tex]E=3(x-a) + a(3-x)+ x(a-3) \\ = 3x - 3a + 3a - ax + ax - 3x \\ = 0[/tex]
[tex]F=(x-6)(x+1)-(x+6)(x-1) \\ {x}^{2} + x - 6x - 6 - ( {x}^{2} - x + 6x - 6) \\ = {x}^{2} - 5x - 6 - {x}^{2} - 5x + 6 \\ = - 10x[/tex]
Bonne journée
