(b) Position of $n^{th}$ maxima from central maxima is given by $x_{n} = \frac{n\lambda D}{d}$
$\Rightarrow x_{n} \propto n\lambda \Rightarrow \frac{d_{1}}{d_{2}} = \frac{n_{1}\lambda_{1}}{n_{2}\lambda_{2}} = \frac{8\lambda_{1}}{6\lambda_{2}} = \frac{4}{3}\left( \frac{\lambda_{1}}{\lambda_{2}} \right)$