$P=\Big(\frac{x^2+3x}{x^3+3x^2+9x+27}+\frac{3}{x^2+9}\Big):\Big(\frac{1}{x-3}-\frac{6x}{x^3-3x^2+9x-27}\Big)$
$(ĐK: x\ne{\pm3})$
$P=\Big(\frac{x(x+3)}{x^2(x+3)+9(x+3)}+\frac{3}{x^2+9}\Big):\Big(\frac{1}{x-3}-\frac{6x}{x^2(x-3)+9(x-3)}\Big)$
$P=\Big(\frac{x(x+3)}{(x^2+9)(x+3)}+\frac{3}{x^2+9}\Big):\Big(\frac{1}{x-3}-\frac{6x}{(x^2+9)(x-3)}\Big)$
$P=\Big(\frac{x}{x^2+9}+\frac{3}{x^2+9}\Big):\Big(\frac{1(x^2+9)-6x}{(x^2+9)(x-3)}\Big)$
$P=\frac{x+3}{x^2+9}:\frac{x^2+9-6x}{(x^2+9)(x-3)}$
$P=\frac{x+3}{x^2+9}.\frac{(x^2+9)(x-3)}{(x-3)^2}$
$P=\frac{x+3}{x^2+9}.\frac{x^2+9}{x-3}$
$P=\frac{x+3}{x-3}$
