What is the surface area of a cone-shaped part with a circumference of 18.84dm and a height of 15dm, melted into a cylinder with the same base as it

What is the surface area of a cone-shaped part with a circumference of 18.84dm and a height of 15dm, melted into a cylinder with the same base as it

The height of a cylinder with equal volume and equal bottom area is one third of that of a cone
15 × 1 / 3 = 5 (decimeter)
What is the radius of the bottom
18.84 △ 3.14 △ 2 = 3 (decimeter)
What's the bottom area
3 × 3 × 3.14 = 28.26 (square decimeter)
What is the lateral area
18.84 × 5 = 94.2 (square decimeter)
What is the surface area
28.26 × 2 + 94.2 = 150.72 (square decimeter)