A parallelogram inscribed in a circle is always a rectangle.

I'll assume you have proved that using the fact that equal chords
subtend equal arcs:



We know that the green line is 5cm because it is a radius.
We know that the red line is 3cm because it is half-way across
the rectangle which has side 6cm.

The answer will be the rectangle's height or 2x.

We find x by the Pythagorean theorem:

3² + x² = 5²
 9 + x² = 25
     x² = 16
      x = 4cm

So the rectangle's height is 2x or 8cm.

Edwin