Answer: The required expression of area is 12x.
Step-by-step explanation: Given that a rectangular window has a length of 12 ft and a width of x ft.
We are to find the expression that represents the area of the window.
We know that
if the length and width of a rectangle are represented by l and w, then the area of the rectangle is given by

For the given rectangular window, we have

Therefore, the area of the window is given by

Thus, the required expression of area is 12x.