MHB Express rectangle area as function of x

[kbillsy]

Hey so another expressing functions question:

A rectangle has on corner on the graph of y=36-x^2, another at the origin, a 3rd on the positive y-axis, and the fourth on the positive x-axis. Express the area A of the rectangle as a function of x. What is the domain of A? For what value of x is A largest?

 

[MarkFL]

Let's look at a diagram:

View attachment 2132

In terms of $x$, what is the base of the rectangle...what is its height?