In this question, the length of a rope is given. The wave pattern established in the rope is shown. You must determine the wavelength from these two pieces of data. To do so, you will need to relate the number of waves shown in the pattern to the length.
Perhaps the easiest way to determine the number of waves is to know what a half-wave looks like. The diagram at the right displays one-half of a wave. One complete wave consists of two halves - the half with the crest and the half with the trough. So simply count the number of halves and divide the result by two in order to determine the number of waves in the pattern.
Once done, you can write an equation of the form N•W = L where N is the number of waves (determined from previous paragraph), W is the wavelength (what you are trying to calculate), and L is the given length. Use the equation to solve for W. You will have to divide L by N to determine the wavelength.