The length of a side must be less than the sum of the other two sides. Here, that means x < 10+13, or x < 23.
In addition, remember that the length of a side must be greater than the difference of the other two sides. That means x > 13 - 10, or x > 3.
Therefore we have two limits for the value of x: 3 and 23.
3 < x < 23