A) 12 - 6x² = (2x +3)(5-3x)
12- 6x² = 10x - 6x² + 15 - 9x
12- 6x² = -6x² + x + 15
-12x² + x - 3 = 0 | * (-1)
12x² - x + 3 = 0
D = 1 - 4*12*3 = 1 - 144 = -143 < 0, корней нет
б)(x+7)(x-8) - (x-9)(x+10) = 0
(x²- 8x + 7x - 56) - (x² + 10x - 9x - 90) = 0
(x² - x - 56) - (x² + x - 90) = 0
x² - x - 56 - x² - x + 90 = 0
-2x + 34 = 0
-2x = -34
x = -34/-2
x = 17