Wavelength is the length of the wave that includes 1 full crest and 1 full trough. If you only have 1/2 a wave in a certain distance, then the full wavelength of that wave should be 2x that distance. For example, in the 3rd picture at the bottom (Standing Waves) - if you were 6 meters apart, how long is each wave? Well, let's start with the middle one. You have 1 crest and 1 trough. Thus you have a whole wave ... in 6m. So the wavelength would be ... 6m. The one above it only has 1/2 a way in 6 meters so ....? The one below it has 1.5 waves in 6 meters, so