The wave function (in S.I. units) for an electromagnetic wave is given as:

            $\psi (x, t) = 10^3 sin\pi (3 \times 10^6 x – 9 \times 10^{14}t)$ , wavelength of the wave is: 

666 nm      

Comparing  $\psi (x, t) = 10^3 sin\pi (3 \times 10^6 x – 9 \times 10^{14}t)$  with 

$\psi (x, t) = a sin\frac{2\pi}{\lambda} (x -vt)$

$\frac{2\pi}{\lambda} = \pi \times 3\times 10^6 $

$ \Rightarrow \lambda = 666nm$