If the width of the cardboard = x cm then its length is given by x + 8 cm.
Then the length of the box will be x + 8 - 2(2) = x + 4
and the width = x - 2(2) = x - 4 cm The height is 2 cm
Volume = h*w*l = 2(x - 4)(x + 4) = 256
2(x^2 - 16) = 256
x^2 - 16 = 128
x^2 = 144
x = 12 cm
Dimensions of the box are:-
length = 12 + 4 = 16cm
width = 12 - 4 = 8 cm
height = 2 cm
