Упрости выражение: a) (6-x)/(x²-9) - (3x-x²)/3, б) (x-2)/1 - (x³-8)/(6x), в) x/3 + (x²-7x)/21 - (x-7)/(4-x).

a) $\frac{6-x}{x^2-9} - \frac{3x-x^2}{3} = \frac{6-x}{(x-3)(x+3)} - \frac{x(3-x)}{3} = \frac{-(x-6)}{(x-3)(x+3)} - \frac{-x(x-3)}{3} = -\frac{x-6}{(x-3)(x+3)} + \frac{x(x-3)}{3} = \frac{-3(x-6) + x(x-3)(x+3)}{3(x-3)(x+3)} = \frac{-3x+18 + x(x^2-9)}{3(x-3)(x+3)} = \frac{-3x+18 + x^3-9x}{3(x-3)(x+3)} = \frac{x^3 - 12x + 18}{3(x-3)(x+3)}$ б) $\frac{x-2}{1} - \frac{x^3-8}{6x} = \frac{x-2}{1} - \frac{(x-2)(x^2+2x+4)}{6x} = \frac{(x-2)6x - (x-2)(x^2+2x+4)}{6x} = \frac{(x-2)(6x - x^2-2x-4)}{6x} = \frac{(x-2)(-x^2+4x-4)}{6x} = \frac{-(x-2)(x^2-4x+4)}{6x} = -\frac{(x-2)(x-2)^2}{6x} = -\frac{(x-2)^3}{6x}$ в) $\frac{x}{3} + \frac{x^2-7x}{21} - \frac{x-7}{4-x} = \frac{x}{3} + \frac{x^2-7x}{21} + \frac{x-7}{x-4} = \frac{7x + x^2 - 7x}{21} + \frac{x-7}{x-4} = \frac{x^2}{21} + \frac{x-7}{x-4} = \frac{x^2(x-4) + 21(x-7)}{21(x-4)} = \frac{x^3 - 4x^2 + 21x - 147}{21(x-4)}$