The area of a rectangle is equal to the product of its length and width.
In this case, the length of the rectangle is the difference in x-coordinates between points C and D, which is x - 0 = x.
The width of the rectangle is the difference in y-coordinates between points B and C, which is 4 - 0 = 4.
So, the area of the rectangle is x * 4 = 28.
Dividing both sides of the equation by 4, we have x = 28/4 = 7.
Therefore, the value of x is $\boxed{7}$.The answer is: 7
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
If the area of rectangle ABCD with vertices A(0, 0), B(0, 4), C(x, 4), and D(x, 0) is 28 square units, and x is greater than 0, what is the value of x?

### Answer:
