In this Kata, you will be given an array of integers and your task is to return the number of arithmetic progressions of size 3 that are possible from that list. In each progression, the differences between the elements must be the same. All inputs will be sorted
solve([1,2,3,5,7,9]) = 5, as follows: [1,2,3], [1,3,5],[1,5,9],[3,5,7], and [5,7,9].