#### Answer

$(r+3w)(r-3t)$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
Group the terms of the given expression, $
r^2-9tw+3rw-3rt
,$ such that the factored form of the groupings will result to a factor common to the entire expression. Then, factor the $GCF$ in each group. Finally, factor the $GCF$ of the entire expression.
$\bf{\text{Solution Details:}}$
Grouping the first and third terms and the second and fourth terms, the given expression is equivalent to
\begin{array}{l}\require{cancel}
(r^2+3rw)-(9tw+3rt)
.\end{array}
Factoring the $GCF$ in each group results to
\begin{array}{l}\require{cancel}
r(r+3w)-3t(3w+r)
\\\\=
r(r+3w)-3t(r+3w)
.\end{array}
Factoring the $GCF=
(r+3w)
$ of the entire expression above results to
\begin{array}{l}\require{cancel}
(r+3w)(r-3t)
.\end{array}