Sentras are 'unibody' vehicles, which means that the body effectively IS the frame. When a unibody FWD car like the Sentra is built, the 'shell' is welded together. Within that shell, structural material is added to support the rear suspension. To finish the structure of the car, a front 'sub-frame' that locates the engine and front suspension is then bolted to the unibody. The rust that you show has already started to 'eat into' that rear supporting structure. At this point, there's nothing that you can do to save it that's financially realistic. You might be able to drive the car as it is for a while, keeping its weakened structure in mind. If the car runs and drives well and you don't mind doing the work, you also might look for another Sentra the same year with a solid body and worn out engine - and swap over the driveline. Otherwise, I'd do what what MikeCStig suggests and part it out. Craigslist is your friend!
Sorry for the bad news...
And - to MikeCStig - Kentucky doesn't have annual inspections. Tommyraym could probably keep it on the road as long as it can pull itself from Point A to Point B. Not an ideal - or particularly safe - solution; but then, I live across the river in Ohio and we have the same set-up.
DB