a, 9x^2 - 6x - 3 = 0
=> 9x^2 - 9x + 3x - 3 = 0
=> (9x^2 - 9x) + (3x - 3) = 0
=> 9x.(x - 1) + 3.(x - 1) = 0
=> (x - 1).(9x + 3) = 0
=> x - 1 = 0 hoặc 9x + 3 = 0
=> x = 1 hoặc 9x = -3
=> x = 1 hoặc x = -3/9 = -1/3
b, x(x + 5)(x - 5) - (x + 2)(x^2 - 2x + 4) = 3
=> x.(x^2 - 25) - (x^3 + 8) = 3
=> x^3 - 25x - x^3 - 8 - 3 = 0
=> -25x - 11 = 0
=> -25x = 11
=> x = -11/25