-2m (m + n - 4) +5 ( -2m +2n) + n ( m +4n - 5)

You just need to solve for g, multiply it out to get -g+6+2g=-4g-4, subtract 6 from both sides to get -g+2g=-4g-10, then add 4g to both sides for -g+2g-4g=-10, then add the g's together to get -3g=-10, divide both sides by -3 and you get g=3,1,3 (three and one third).