Hello, NeedHelp18!
Did you make a sketch?
A rectangle has a vertex on the graph of $\displaystyle y \:= \:9  x^2$,
one at the origin, one on the positive x  axis, and the fourth one on the positive y  axis.
(a) Express the area of the rectangle as a function of $\displaystyle x.$ Code:

*9
*  *
* +   *
* :::::: *
* ::::::y *
::::::
*++*
 x 3
The area of the rectangle is: .$\displaystyle A \:=\:xy\text{, where }y \:=\:9x^2$
Therefore: .$\displaystyle A(x) \;=\;x(9x^2)$
The domain is: .$\displaystyle 0 \,\leq\, x \,\leq\,3$
(c) What is the area of the rectangle when $\displaystyle x = 1.5$? $\displaystyle A(1.5) \;=\;1.5(9  1.5^2) \;=\;10.125$