Answer
$1.98\times10^{-19}\,J$
Work Step by Step
For Paschen series, $n_{f}=3$. Given, $n_{i}=7$.
$\Delta E=-(2.18\times10^{-18}\,J)(\frac{1}{n_{f}^{2}}-\frac{1}{n_{i}^{2}})$
$=-(2.18\times10^{-18}\,J)(\frac{1}{3^{2}}-\frac{1}{7^{2}})$
$=-(2.18\times10^{-18}\,J)(\frac{1}{9}-\frac{1}{49})$
$=-1.98\times10^{-19}\,J$
Negative sign indicates that the energy is emitted.
