A trapezium $\text{ABCD}$ has side $\text{AD}$ parallel to $\text{BC}, \angle \text{BAD}=90^{\circ}, \text{BC}=3 \text{~cm}$ and $\text{AD}=8 \text{~cm}$ . If the perimeter of this trapezium is $36 \text{~cm}$ , then its area, in sq. $\text{cm}$ , is