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The volume of a cylinder is given by where r is the radius of the cylinder and h is the height.
You are told that the height of the cylinder is 12 in so the 18 in edge of the paper must form the circumference of the cylinder. Circumference is given by so you must have . Solve that for r and use h= 12.
Now, how did you get ? We can't tell what you did wrong if you don't tell us what you did!