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