circumference is the diameter × π

diameter is radius × 2

first circle ->

r = 9

d = 9 × 2 = 18

c = 18 × π ≈ 56.52

second circle ->

r = 16

d = 16 × 2 = 32

c = 32 × π ≈ 100.48

so the ratio of circurfremce for 2 circles that have a ratio of radii 9:16 would be 56.52:100.48