$\text{ABCD}$ is a trapezium in which $\overline{\text{AB}}\parallel \overline{\text{CD}}$. $\text{P}$ and $\text{Q}$ are the mid - points of

$\overline{\text{AD}}$ and $\overline{\text{BC}}$ respectively. If $\text{AB}=8cm, \text{PQ}=6cm$, then $\text{CD}=$