Answer
$(4+x+3y)(4-x-3y)$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To factor the given expression, $
16-(x+3y)^2
,$ use the factoring of the difference of $2$ squares.
$\bf{\text{Solution Details:}}$
Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of $2$ squares, the factored form of the expression above is
\begin{array}{l}\require{cancel}
[4+(x+3y)][4-(x+3y)]
\\\\=
[4+x+3y][4-x-3y]
\\\\=
(4+x+3y)(4-x-3y)
.\end{array}