math magic

Have you tried making magic?
How about Math Magic?
Would you like to try this and reveal the magic?
Here it is.....

Let somebody think or choose any number.
Let him do the following

Add 6 to the number (the number he choose).
Multiply the sum by 2.
Subtract the number from the product twice.

Tell him that the final answer is 12.
Surprise?
How did the magic works?
Wanna tell me?