Answer
$A(x)=\dfrac{x^2}{2}$ where $x$ is the length of the rectangle.
Work Step by Step
The area of the rectangle has to be a function of the rectangle's length.
Thus, we need to let $x$ be the length.
Since the length is twice the width and the length is $x$, then the width is $\frac{x}{2}$.
Recall that the area $A$ of a rectangle is equal to the product of its length and width,
Hence,
The area is $A=x\left(\dfrac{x}{2}\right)=\dfrac{x^2}{2}$
