Required area
$=\pi \times(6)^{2}-2 \int\limits_{0}^{3} \sqrt{9} x d x-\int\limits_{3}^{6} \sqrt{36-x^{2}} d x$
$=36 \pi-12 \sqrt{3}-2\left(\frac{ x }{2} \sqrt{36- x ^{2}}+18 \sin ^{-1} \frac{ x }{6}\right)^{6}$
$=36 \pi-12 \sqrt{3}-2\left(9 \pi-3 \pi-\frac{9 \sqrt{3}}{2}\right)$
$=24 \pi-3 \sqrt{3}$