Answer
$(a-15b)(a-3b)$
Work Step by Step
$(a+mb)(a+nb)=a^{2}+(m+n)ab+(mn)b^{2}$
$a^{2}-18ab+45b^{2}=$
Searching for two factors of $+45$ whose sum is $-18,$
we find$\qquad-15$ and $-3$
= $(a-15b)(a-3b)$
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