Yesterday, the best answer is: -2x^2-2x.  (Contributed by fire. Good Job!)
dnialh solved the puzzle at 20:09:21.322; length: 17
dnialh solved the puzzle at 20:11:51.709; length: 15
dnialh solved the puzzle at 20:13:01.623; length: 11
dnialh solved the puzzle at 20:13:52.268; length: 11
fire solved the puzzle at 20:16:17.252; length: 11
dnialh solved the puzzle at 20:17:06.252; length: 11