\(\Rightarrow \frac{n+1}{n}=\frac{420}{315} \Rightarrow n=3\)
Hence \(3 \times \frac{v}{2 \ell}=315 \Rightarrow \frac{v}{2 \ell}=105 \mathrm{Hz}\)
Lowest resonant frequency is when \(n=1\)
Therefore lowest resonant frequency \(=105 \mathrm{Hz}\)
$(a)$ $\left(x^2-v t\right)^2$
$(b)$ $\log \left[\frac{(x+v t)}{x_0}\right]$
$(c)$ $e^{\left\{-\frac{(x+v t)}{x_0}\right\}^2}$
$(d)$ $\frac{1}{x+v t}$