Answer
The wavelength of wave A is $\lambda_A=35.56nm$
The wavelength of wave B is $\lambda_B=80nm$
Work Step by Step
The wavelength of a wave is the distance between 2 adjacent peaks (or 2 adjacent troughs) of that wave.
*Wave A:
We would call the distance between A and the point where the red line passes the black line for the first time by $a$.
We can see that $$9a=1.6\times10^{-7}m$$$$a\approx1.778\times10^{-8}m$$
Now we measure the distance between 2 adjacent peaks, which is the wavelength of wave A $(\lambda_A)$.
We also find that $$\lambda_A=2a=2\times(1.778\times10^{-8}m)$$$$\lambda_A=3.556\times10^{-8}m=35.56nm$$
*Wave B:
Similary, we call the distance between B and the point where the red line passes the black line for the first time by $b$.
We can see that $$4b=1.6\times10^{-7}m$$$$b=4\times10^{-8}m$$
Now we measure the distance between 2 adjacent peaks, which is the wavelength of wave B $(\lambda_B)$.
We also find that $$\lambda_B=2b=2\times(4\times10^{-8}m)$$$$\lambda_B=8\times10^{-8}m=80nm$$
