(x+4)*(x+1) = (x-1)(x-6) =>produit en croix
x²+x+4x+4 = x²-6x-x+6
x²+5x+4=x²-7x+6
x²+5x+7x -x² = 6 - 4
12x = 2
x = 2/12
x = 1/6
