on comparing with standard equation of stationary wave
$y=R \sin \frac{2 \pi \mathrm{x}}{\lambda} \cdot \sin \omega \mathrm{t},$ we get
$\frac{2 \pi \mathrm{x}}{\lambda}=\mathrm{bx}$
$\lambda=\frac{2 \pi}{b}$
The distance between constructive nodes
$=\frac{\lambda}{2}=\frac{2 \pi / \mathrm{b}}{2}=\frac{\pi}{\mathrm{b}}$
