Line 1: (2x + 6x) + 4y + 3x - 2xy
Line 2: (8x) + 4y + 3x - 2xy
Line 3: 8x + 3x + 4y - 2xy
Line 4: 11x + 4y - 2xy
Line 5: 13xy
It's all perfectly correct until line 5.
11x + 4y - 2xy ,is as simplified as this equation can get. You cannot simply add all the terms disregarding if they are 'x's, 'y's or 'xy's