x = length of third side
The third side is unknown and can't be determined to be one fixed number. However, we can find a set of values or a range of acceptable values
If we know that
a = 6
b = 7
are the two given sides, then we can say
b-a < x < b+a
7-6 < x < 7+6
1 < x < 13
So the third side x can be between 1 and 13.
The third side cannot be equal to 1 unit or 13 units because a triangle would not form. Instead a straight line would form.
Note: This idea is using the triangle inequality theorem