3.64: Shaded region with parallel chords and arcs

The diagram shows the shaded region $ABCD$. Lines $AC$ and $BD$ each have length $12\text{ cm}$ and bisect each other at $O$. Lines $AD$ and $BC$ are parallel and each have length $4\text{ cm}$. Arcs $AB$ and $DC$ are part of a circle centre $O$.