P = \(\left(\dfrac{x+3}{3-x}-\dfrac{3-x}{x+3}-\dfrac{4x^2}{9-x^2}\right):\dfrac{2x+1-x-3}{x+3}\)
= \(\left[\dfrac{\left(x+3\right)^2-\left(x-3\right)^2-4x^2}{\left(3-x\right)\left(x+3\right)}\right]:\dfrac{x-2}{x+3}\)
= \(\dfrac{x^2+6x+9-x^2+6x-9-4x^2}{\left(3-x\right)\left(x+3\right)}.\dfrac{x+3}{x-2}\)
= \(\dfrac{-4x^2+12x}{\left(3-x\right)\left(x+3\right)}.\dfrac{x+3}{x-2}\)
= \(\dfrac{4x\left(3-x\right)}{\left(3-x\right)\left(x-2\right)}=\dfrac{4x}{x-2}\)
\(P=\left(\dfrac{3+x}{3-x}-\dfrac{3-x}{3+x}-\dfrac{4x^2}{9-x^2}\right):\left(\dfrac{2x+1}{x+3}-1\right)\)
\(P=\left(\dfrac{\left(3+x\right)^2-\left(3-x\right)^2+4x^2}{9-x^2}\right):\left(\dfrac{2x+1-x-3}{x+3}\right)\)
\(P=\left(\dfrac{9+6x+x^2-9+6x-x^2+4x^2}{\left(x-3\right)\left(x+3\right)}\right):\left(\dfrac{x-2}{x+3}\right)\)
\(P=\dfrac{4x^2+12x}{\left(x-3\right)\left(x+3\right)}.\dfrac{x+3}{x-2}\)
\(P=\dfrac{4x\left(x-3\right)}{x-3}.\dfrac{1}{x-2}\)
\(P=\dfrac{4x}{x-2}\)