Answer
$\dfrac{3\pi}{4}$
Work Step by Step
Consider $x=3 \sin \theta \implies dx=3 \cos \theta d\theta$
Now, the given integral can be written as:
$Area=\int_0^{\pi/2} \dfrac{\sqrt{9-9 \sin^2 \theta}}{3} d \theta=\int_0^{\pi/2} 3 \cos^2 \theta d\theta$
or, $=\dfrac{3}{2}[\theta+(1/2) \sin 2 \theta]_0^{\pi/2}+C$
Now, Area $=\dfrac{3}{2}(\dfrac{\pi}{2})=\dfrac{3\pi}{4}$
