${f_2} - {f_1} = \frac{{35}}{{24}} \times {10^{14}}$
$3 \times {10^8}C(\frac{1}{{{\lambda _2}}} - \frac{1}{{{\lambda _1}}}) = \frac{{35}}{{24}} \times {10^{14}}$
$\frac{1}{{{\lambda _2}}} - \frac{1}{{{\lambda _1}}} = \frac{{35}}{{72}} \times {10^6} \times {10^{ - 9}} = \frac{{35}}{{72}} \times {10^{ - 3}}$
$\frac{1}{\lambda } = \frac{1}{{100}}(\frac{1}{{{{n'}^2}}} - \frac{1}{{{n^2}}}) \to \frac{1}{{{\lambda _2}}} - \frac{1}{{{\lambda _1}}} = \frac{1}{{100}}(\frac{1}{{{{(n + 1)}^2}}} - \frac{1}{{{{(n + 2)}^2}}}) = \frac{{35}}{{72}} \times {10^{ - 3}}$
$\frac{1}{{{{(n + 1)}^2}}} - \frac{1}{{{{(n + 2)}^2}}} = \frac{7}{{144}} = \frac{7}{{9 \times 16}} \to \left\{ \begin{gathered}
n + 1 = 3 \hfill \\
n + 2 = 4 \hfill \\
\end{gathered} \right. \to n = 2$