1.
(x - x1)(x - x2) = 0
(x - 6)(x + 9) = 0
x² + 9x -6x - 54 = 0
x² + 3x - 54 = 0
2.
теорема Виета
x² + px + q = 0
x1 + x2 = -p
x1*x2 = q
x1 + x2 = -9 + 6 = -3
x1*x2 = 6*(-9)= -54
x² + 3x - 54 = 0
(x-6)(x+9)=0
x²+9x-6x-54=0
x²+3x-54=0
использовал (x-x₁)(x-x₂)=0
