$$m^{2}-18m+81-n^{2}$$
$\left(9-n-m\right)\left(9+n-m\right)$
$$\left(-9+m-n\right)\left(-9+m+n\right)$$
$$\left(m-n-9\right)\left(m+n-9\right)$$
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$\left(m-9\right)^{2}-n^{2}$
