What is the range of possible sizes for side x? the other two sides are 8.0 and 8.5

I assume we are talking about a triangle here.

Let a=8 and b=8.5

Let the angle between a and b be θ

Let the unknown side be c

As θ → 0, c → 0.5

As θ → 180∘, c → 16.5

So the lenght of side c will be in the interval:

0.5