A string has a tension of 72 N and a mass of 0.02 kg for each metre.

Step 1. (a) $v = \sqrt{\dfrac{72\ \text{N}}{0.02\ \text{kg/m}}} = \sqrt{3600}\ \text{m/s} = 60\ \text{m/s}$.

Step 2. (b) $\lambda = \dfrac{60\ \text{m/s}}{30\ \text{Hz}} = 2\ \text{m}$ and $\dfrac{60\ \text{m/s}}{60\ \text{Hz}} = 1\ \text{m}$.