The volume is given in terms of length (L), width (W), and height (H) by
... V = LWH
Fill in the given information to get
... 72 m³ = (6 m)·W·(3 m)
Divide by the coefficient of W to get
... W = (72 m³)/(18 m²) = 4 m
The width of the prism is 4 meters.