$${ \left(x-3y \right) }^{ 2 } -9 { \left(x-3y \right) }^{ 2 }$$
$-8\left(x-3y\right)^{2}$
$$x^{2}-6xy+9y^{2}-9\left(x-3y\right)^{2}$$
$$x^{2}-6xy+9y^{2}-9\left(x^{2}-6xy+9y^{2}\right)$$
$$x^{2}-6xy+9y^{2}-9x^{2}+54xy-81y^{2}$$
$$-8x^{2}-6xy+9y^{2}+54xy-81y^{2}$$
$$-8x^{2}+48xy+9y^{2}-81y^{2}$$
$$-8x^{2}+48xy-72y^{2}$$
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$-8x^{2}+48xy-72y^{2}$
