Answer
$A(x)=\frac{9x-x^3}{2}.$
Work Step by Step
We know that at for a right triangle $A=\frac{\text{length}⋅\text{width}}{2}$, where $A$ is the area.
Hence here the width is $x$, the length is $y=9-x^2$, therefore $A(x)=\frac{x\cdot(9-x^2)}{2}=\frac{9x-x^3}{2}.$
