(b) Let the equation of wave be $y = A\sin{(\omega t - kx)}$
Where $\omega = 2\ \pi\ n\ and\ k = \frac{2\pi}{\lambda}$
Wave velocity, $V = n\lambda = \frac{\omega}{2\pi} \times \frac{2\pi}{k} = \frac{\omega}{k}$
Maximum particle velocity $v = A\omega$
For $V = v;\frac{\omega}{k} = A\omega\ or\ A = \frac{1}{k} = \frac{\lambda}{2\pi}$