c = ƒλ

λ = c/f

λ = 3 x 10^8 / 2.05 x 10^9

λ = 0.146 m

c = speed of light in a vacuum since this is part of the electromagnetic spectrum and these share same velocity in a vacuum. I approximated the velocity in a vacuum and we take the speed in the vacuum as the same as the speed in air.

c = lambda x v

lambda = c / v

c = 3.00x10^8 m/s

The SI unit of frequency s^-1 is hertz

lambda = 3.00x10^8 m/s / (2.05 ✕ 10^9 s^-1) = 0.146 m

1 nm = 1x10^9 m

simply multiply by 1x10^9

0.146 m x 1x10^9 = 1.46x10^8 nm