Answer
$49m^2-64n^2$.
Work Step by Step
The given expression is
$=(7m+8n)(7m-8n)$
Use sum and difference pattern $(a+b)(a-b)=a^2-b^2$.
We have $a=7m$ and $b=8n$.
$=(7m)^2-(8n)^2$
Simplify.
$=49m^2-64n^2$
Hence, the product is $49m^2-64n^2$.