Final answer:
The length of the rectangular slab of concrete is x/2 feet.
Step-by-step explanation:
To find the length of the rectangular slab of concrete, we first need to find its area. The area is given as square feet, so we can represent it as x^2. The width of the slab is given as 2x feet, so the length can be found by dividing the area by the width:
Length = Area / Width = x^2 / 2x = x/2
Therefore, the length of the rectangular slab is x/2 feet.