Let s denote the slanted height, r the radius of the base circle and h the height of the cone. Then the curved surface of a cone is a sector of a circle with radius s. The area of the curved surface, the area of the complete circle, the circumference of the base circle = arc of the sector and the circumference of the complete circle are directly proportional:
Hello GAdams
earboth has given you the formula for the curved surface area of a cone. You now need to work out the value of , which you can do using Pythagoras' Theorem. (It's the easiest right-angled triangle of all!)
Then plug the values of and into this formula, and you're there.
Grandad