You are trying to solve:

12=6+6-2(6)

The above expression does not have a variable.

Since there is no variable in the expression there is no solution.

If you simply calculate: 6+6-2(6) you will get 

6+6-2(6) = 0

click here to see the step by step calculation

......

This means that in the end you will indeed get:

12 = 0

12 = 0 is a false statement.