Check the picture below.
the idea being that sides of 2.5 andd 8 are fixed, what's are the possible lengths of "x"?
well, if we open the fixed sides, we'll end up with a 3rd side of 2.5 + 8 = 10.5, however if "x" is ever 10.5, the figure will become a flat-line and we'd be losing the triangle.
and if we close the fixed sizes, the longest "x" could be is 8 - 2.5 = 5.5, but the same, it can't be 5.5 exactly otherwise the figure will turn to a flat-line.
5.5 < x < 10.5.