Answer
$(x+3y)(x-6y)$
Work Step by Step
The given expression, $
x^2-3xy-18y^2
,$ has $c=
-18
$ and $b=
-3
.$
The possible factors of $c$ are $
\{ 1,-18 \}
,\{ 2,-9 \}
,\{ 3,-6 \}
,\{ -1,18 \}
,\{ -2,9 \}
,\{ -3,6 \}
$. Among these factors, the pair whose sum is equal to $b$ is $\{
3,-6
\}.$ Hence, the factored form is
\begin{array}{l}\require{cancel}
(x+3y)(x-6y)
.\end{array}