A trapezoid has one base equal to its height, $x$, and the other base is twice as long. Write the expression for the area of the trapezoid as a common fraction in terms of $x$.
The area of a trapezoid is equal to the product of the height and the average of the length of the bases. In this case, since the length of the two bases are $x$ and $2x$ and the length of the height is $x$, the area is equal to $\frac{x+2x}{2} \cdot x=\frac{3x}{2}\cdot x=\boxed{\frac{3x^2}{2}}$.