Question 944553
The unknown side length, x.


Use Triangle Inequality Theorem.
{{{system(9+20>x,9+x>20,20+x>9)}}};
Solve for the possible solutions of x within this system.  The result will be an inequality, NOT a specific value.



The last inequality of the system will give nonsense.
The other two inequalities of the system will give you  {{{highlight(11<x<29)}}}.