Answer
$\dfrac{9x-x^3}{2}$
Work Step by Step
The area for a right triangle is given by: $A=\dfrac{\text{length} \times \text{width}}{2}$
where $A$ is the area.
We are given that the width is $x$ and the length is $y=9-x^2$.
Thus, we have:
$A(x)=\dfrac{(x)(9-x^2)}{2}=\dfrac{9x-x^3}{2}$