a. (2x-3)(x²-x+3)
= 2x³-2x²+6x-3x²+3x-9
= 2x³-5x²+9x-9
<=> x(2x²-5x+9) = 9
<=> x=9 hoặc 2x²-5x+9=9
<=> x=9 hoặc x= 5/4
b. (x+2)³-(x-1)³-9x(x+1)
= x³ + 6x² + 12x + 8 - (x³ - 3x² + 3x - 1) - (9x² + 9x)
= x³ + 6x² + 12x + 8 - x³ + 3x² - 3x + 1 - 9x² - 9x
= 9