Answer
$(m-3)(m+n)$.
Work Step by Step
The given polynomial is
$=m^2-3m+mn-3n$
Group the terms.
$=(m^2-3m)+(mn-3n)$
Factor each group.
$=m(m-3)+n(m-3)$
Factor out $(m-3)$.
$=(m-3)(m+n)$
Hence, the complete factor of the polynomial is $(m-3)(m+n)$.
