$$(x+3)^{2}-(x-3)^{2}\cdot(x+3)=$$
$\left(1-x\right)\left(x-6\right)\left(x+3\right)$
$$x^{2}+6x+9-\left(x-3\right)^{2}\left(x+3\right)$$
$$x^{2}+6x+9-\left(x^{2}-6x+9\right)\left(x+3\right)$$
$$x^{2}+6x+9-\left(x^{3}-3x^{2}-9x+27\right)$$
$$x^{2}+6x+9-x^{3}+3x^{2}+9x-27$$
$$4x^{2}+6x+9-x^{3}+9x-27$$
$$4x^{2}+15x+9-x^{3}-27$$
$$4x^{2}+15x-18-x^{3}$$
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$-x^{3}+4x^{2}+15x-18$