(4x² - 7x + by - 1) - (-2x² + 3y² + 2y + 2x）+（-6x² + 9x - y)
= 4x² - 7x + by - 1 + 2x² - 3y² - 2y - 2x - 6x² + 9x - y
= -3y² + (b - 3)y - 1
所以值与 x 无关.