Combine the like terms to make a simpler expression:
8n + 12 + (-9) - (-6n)

8n + 12 + (-9) - (-6n)

8n + 12 + -9 + -1 × -6n 

8n + 12 + -9 + 6n

12 + -9 + 8n + 6n
3 + 8n + 6n

3 + 14n

8n + 12 + (-9) - (-6n) simplified = 3 + 14n 

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.