Any value equal to less than 2.5 or greater than or equal to 13.5
The two smaller sides of a triangle have to have a sum larger than the longest side. so if 8 is the longest side 8-5.5=2.5 or the value or below it cannot be. If 8 is not the longest side then 8+5.5=13.5 is the value the unknown side has to be smaller than.