#### Answer

$(7m-6n)^2$

#### Work Step by Step

The first term is $7m$ squared, the last term is $6n$ squared, and the middle term is twice the negative product of these two terms. Now we can factor the trinomial into a binomial squared.
$49m^2-84mn+36n^2$
$=(7m-6n)(7m-6n)$
$=(7m-6n)^2$