By definition, we have to
W = V * Dw
W: weight
V: Volume
D: Density
Clearing and Substituting we have:
V = W / Dw = (3.2 lb) / (0.021 (lb / in ^ 3)) = 152.38 in ^ 3
Then, on the other hand, the volume is
V = l * w * d
l: long
w: width
d: depth
Clearing and substituting
l = V / (w * d) = (152.38 in ^ 3) / ((1.5 in) * (5.5 in)) = 18.5 in
The answer is 18.5 in