Answer
(m-7n)(m+4n)
Work Step by Step
You are given $m^{2}$-3mn-28$n^{2}$. Factor it out:
$m^{2}$-7mn+4mn-28$n^{2}$ -break out the middle value-
($m^{2}$-7mn)+(4mn-28$n^{2}$) -group the first two and last two terms-
m(m-7n)+4n(m-7n) -Find the GCF of each grouping-
(m+4n)(m-7n) -factor out the common term-
=(m+4n)(m-7n)