The answer is -2. Since you want to leave your x alone with the number you have to move the 6 to the other side. Now it has become a -6 rather than a 6. It is now -3x=6. (It is 6 because since you removed 6 from the 12). You now wish to leave the x alone so you have to divide by -3. (-3x/-3=6/-3). It should equal to -2. Remember that it is a negative number you are dividing by so you should make the answer negative since they are different signs!
