Answer
Length: 36 feet
Width: 18 feet
Height: 1/3 feet
Work Step by Step
The volume of a prism is determined by $l\cdot w\cdot h$
The volume of the patio floor is 8 cubic yards or 216 cubic feet
The height is $\frac{1}{3}$ feet and the length is twice the width, so $l=2w$
The following equation arises: $2w\cdot w \cdot \frac{1}{3}= 216$
Now, w can be solved:
$w^2=216\cdot \frac{3}{2}$
$w^2=324$
$\sqrt{w^2}=\sqrt{324}$
$w=18$
The width is 18 feet, so the length is 36 feet.