11. (This sub-question is worth 15 points) Divide the set $M=\{1,2, \cdots, 12\}$ of the first 12 positive integers into four three-element subsets $M_{1}, M_{2}, M_{3}, M_{4}$, such that in each three-element subset, one number is equal to the sum of the other two. Find the number of different ways to do this.