12 + 5x = 6 - x
This problem can be solved with the given information, so we should find that no facts are missing:
12 + 5x = 6 - x
Add x to each side:
12 + 5x + x = 6 - x + x
Combine like terms:
12 + 6x = 6
Subtract 12 from each side:
12 - 12 + 6x = 6 - 12
Do the arithmetic:
6x = -6
Divide each side by 6:
x = -1