Question

(7m^{2}-4mn-n{2})-(2m^{2}-mn-n{2})

Answer 1

$displaystyle (7m^{2}-4mn-n^{2})-(2m^{2}-mn-n^{2})= \ =7m^{2}-4mn-n^{2}-2m^{2}+mn+n^{2}=5m^{2}-3mn=m(5m-3n)$