$$(n+12)(n+3)=n(n+9)+84.$$
$n=8$
$$n^{2}+15n+36=n\left(n+9\right)+84$$
$$n^{2}+15n+36=n^{2}+9n+84$$
$$n^{2}+15n+36-n^{2}=9n+84$$
$$15n+36=9n+84$$
$$15n+36-9n=84$$
$$6n+36=84$$
$$6n=84-36$$
$$6n=48$$
$$n=\frac{48}{6}$$
$$n=8$$
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