The '3' in the denominator comes from counting the number of complete wavelengths shown in the diagram over the $3 \, \text{m}$ distance. Step 1: Count the number of complete wavelengths. • A complete wavelength ($\lambda$) consists of one full cycle (e.g., one crest and one trough). • Looking at the diagram: • From $0 \, \text{m}$ to $1 \, \text{m}$, there is $\text{1 complete wavelength}$. • From $1 \, \text{m}$ to $2 \, \text{m}$, there is a $\text{2nd complete wavelength}$. • From $2 \, \text{m}$ to $3 \, \text{m}$, there is a $\text{3rd complete wavelength}$. • So, there are $\text{3 complete wavelengths}$ shown within the total distance of $3 \, \text{m}$. Step 2: Calculate the length of one wavelength. Since $3$ complete wavelengths fit into $3 \, \text{m}$, the length of one wavelength ($\lambda$) is: $$\lambda = \frac{\text{Total distance}}{\text{Number of complete wavelengths}}$$ $$\lambda = \frac{3 \, \text{m}}{3} = 1 \, \text{m}$$