(a) Comparing given equation with standard equation
$y = 2a\sin\frac{2\pi x}{\lambda}\cos\frac{2\pi vt}{\lambda}$ gives us $\frac{2\pi}{\lambda} = \frac{\pi}{15} \Rightarrow \lambda = 30$
Distance between nearest node and antinodes
$= \frac{\lambda}{4} = \frac{30}{4} = 7.5$