The time period of revolution of electron in its ground state orbit...

$T ∝ \dfrac{r}{v} ∝ \dfrac{{n}^{2}}{z} \times \dfrac{n}{z} ∝ \dfrac{{n}^{3}}{{z}^{2}}$
$\dfrac{{T}_{1}}{{T}_{2}} = {n}_{1}^{3}{n}_{2}^{3} = \dfrac{1}{8}$
${T}_{2} = 8{T}_{1}$

now,
${f}_{2 = \dfrac{1}{{T}_{2}}}$
${f}_{2} = \dfrac{1}{12.8 \times {10}^{- 16}} \approx 7.8 \times {10}^{14}$