1.
(x - x1)(x - x2) = 0
(x - 3)(x + 5) = 0
x² + 5x -3x - 15 = 0
x² + 2x - 15 = 0
2.
теорема Виета
x² + px + q = 0
x1 + x2 = -p
x1*x2 = q
x1 + x2 = -5 + 3 = -2
x1*x2 = 3*(-5)= -15
