Both terms have a [tex]-m^2n[/tex] in common, so we can pull it out. The factored form will be [tex]-m^2n[/tex] times whatever is left after pulling it out.
In the first term, we will have just 8 left since everything else in the term is factored out.
The second term has a [tex]7m^4n^3[/tex] left. Notice that both of these terms no longer have a negative sign since the negative was factored out.
The answer is [tex](-m^2n)(8+7m^4n^3)[/tex]
