Alright, 1, 2, and 4 are correct but 3 is not.
The error you made could be hard to notice but it was made. So let's put the equation up, 6x-(2x-9)=-31. Your error occurred when you didn;t distribute the negative sign for (2x-9), if you woud have distributed it then the equation would then be simplified into 6x-2x+9=31. You then add like terms and get 4x+9=31, subtract 9 on both sides and you'll get 4x=22. Divide 4 on both sides to isolate the variable and you get x=5.5
