Angle $ABC$ of $△ABC$ is a right angle. The sides of $△ABC$ are the diameters of semicircles as shown. The area of the semicircle on $\overline{AB}$ equals $4.5\pi$, and the arc of the semicircle on $\overline{BC}$ has length $4\pi$. What is the area of the semicircle on $\overline{AC}$?