Perimeter of upper circular end of frustum = 18

2πr_{1} =18

r_{1 }=9/π

Perimeter of lower end of frustum = 6 cm

2πr_{2} = 6

r_{2}= 3/π

Slant height (l) of frustum = 4

CSA of frustum = π (r_{1} + r_{2}) l

Therefore, the curved surface area of the frustum is 48 cm^{2}.