The length has to be greater than the absolute difference between the other two lengths, but less than the sum of them. In other words, if your unknown side length is a, and the other two are b and c, then its |b - c| < a < b + c
So, putting in your values, you have 10 - 7 < a < 10 + 7, which is 3 < a < 17
So, your range is from 3 to 17 (non inclusive).
