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(d) $\left(x^{2}-12 x+36\right)$

$(x-6)(x-6)$

$\Rightarrow(x-6)^{2}$  (according to the formula $\left.(a-b)^{2}=a^{2}-2 a b+b^{2}\right)$

$\Rightarrow\left(x^{2}\right)-2(x)(6)+(6)^{2}$

$\Rightarrow x^{2}-12 x+36$