#### Answer

$(4x-7y)^2$

#### Work Step by Step

The two numbers whose product is $ac=
16(49)=784
$ and whose sum is $b=
-56
$ are $\{
-28,-28
\}$. Using these two numbers to decompose the middle term, then the factored form of the expression, $
16x^2-56xy+49y^2
$, is
\begin{array}{l}\require{cancel}
16x^2-28xy-28xy+49y^2
\\\\=
(16x^2-28xy)-(28xy-49y^2)
\\\\=
4x(4x-7y)-7y(4x-7y)
\\\\=
(4x-7y)(4x-7y)
\\\\=
(4x-7y)^2
.\end{array}