Points D, K, and R are in phase with point A while point E is in phase with point B.
There are 3 and half waves in 8m.
The wavelength can be calculated as
[tex]\begin{gathered} 3.5\lambda=8 \\ \lambda=\frac{8}{3.5} \\ =2.285\text{ m} \end{gathered}[/tex]It takes 16 seconds to complete 3 oscillations.
The time period can be calculated as
[tex]\begin{gathered} 3T=16\text{ s} \\ T=\text{ }\frac{16}{3} \\ =5.33\text{ s} \end{gathered}[/tex]The frequency of the wave will be
[tex]\begin{gathered} f=\frac{1}{T} \\ =\frac{1}{5.33} \\ =0.19\text{ Hz} \end{gathered}[/tex]The speed of the wave can be calculated as
[tex]\begin{gathered} v=f\lambda \\ =\text{ 0.19}\times2.285 \\ =\text{ 0.434 m/s} \end{gathered}[/tex]