A rectangular metal sheet is 8 cm wide and its length is twice its width. Equal squares are cut from each of the four corners and the sides are folded up to form an open-top box. Express the dimension

Let the width of the rectangular metal sheet be $W$ and its length be $L$.
The problem states that the length is twice its width: $L = 2W$.
It also states that the width of the metal sheet was 8 cm.
So, $W = 8$ cm.
Then, the length of the metal sheet is $L = 2 \times 8 = 16$ cm.

Squares of side length $x$ cm are cut from each of the four corners. When the sides are folded up, these cut squares determine the height of the tank.
Height of the tank, $h = x$ cm.
The length of the base of the tank will be the original length minus $2x$ (since $x$ is removed from both ends):
Length of the base, $l_{tank} = L - 2x = 16 - 2x$ cm.
The width of the base of the tank will be the original width minus $2x$:
Width of the base, $w_{tank} = W - 2x = 8 - 2x$ cm.
Note: For a valid tank to be formed, $x$ must be greater than 0, and both $16-2x$ and $8-2x$ must be positive. This implies $x < 4$. The statement "cutting equal squares of length 6 cm" in the problem description is inconsistent with the width of 8 cm, as $x=6$ would lead to a negative width ($8-2(6) = -4$). Therefore, we treat $x$ as a variable for the cut square side length as requested