Answer
The wavelength is $2.4~km$
Work Step by Step
We can find the speed of the waves:
$v = \frac{d}{t} = \frac{180~km}{30.0~s} = 6.0~km/s$
On the graph, we can see that there are four cycles in $1.6~s$. We can find the period:
$T = \frac{1.6~s}{4~cycles} = 0.40~s$
We can find the wavelength:
$\lambda = \frac{v}{f}$
$\lambda = \frac{v}{1/T}$
$\lambda = v~T$
$\lambda = (6.0~km/s)(0.40~s)$
$\lambda = 2.4~km$
The wavelength is $2.4~km$
