Question

What is the value for x on a triangle if one side is 13 and one side 12

Answer 1

The Value of X is 10. 
KhanAcademy.org has some great explanation videos on their website and they have a YouTube channel. 
Here is an explanation for the answer. 

Answer 2

X could be anything in exclusive range from shortest possible value x = 13-12 = 1 to longest possible value x = 12+13 = 25 

Anything equals or less than 1 would be too short to make triangle.

Likewise, anything equals or greater than 25 would be too long to make triangle.

So x is anything in range (1, 25)