I can't think anymore about Luther and Augustine. They're driving me nuts! I think I finally understand where they differ on the Will, but I sure hope and pray that what I'm writing makes as much sense to my professor as it does to me. Augustine (St) is so slippery sometimes!
To make it very, very simple, I think it's like this.
Augustine: Free Will.
"Salvation by Grace" recipe: God gratuitously gives a "good will" to evil merit, then helps the "good will" with grace so that the "good will" does good works which constitute good merit which is rewarded with eternal life.
Luther: Enslaved Will.
"Salvation by Grace" recipe: God gives faith to believe Gospel of salvation. Faith justifies because Christ's works are vicariously applied, thus fulfilling the law, without any works of the man justified.