Question 1107779
Maybe this is a triangle figure.  Let this unknown side be x.


You use Triangle Inequality Theorem.  I'll set it up for your example but you can solve the rest of the way.


{{{system(8+12>x,8+x>12,x+12>8)}}}; but you will want to focus on the first two inequalities.  (Because you know that x must be a positive real number.)