For a hydrogen atom,the energy change for a transition from $n_{i}=5$ to $n_{f}=2$ is given by the Rydberg formula:
$\Delta E = 2.18 \times 10^{-18} \, J \left( \frac{1}{n_{f}^{2}} - \frac{1}{n_{i}^{2}} \right)$
$\Delta E = 2.18 \times 10^{-18} \, J \left( \frac{1}{2^{2}} - \frac{1}{5^{2}} \right) = 2.18 \times 10^{-18} \left( \frac{1}{4} - \frac{1}{25} \right) = 2.18 \times 10^{-18} \left( \frac{21}{100} \right) = 4.578 \times 10^{-19} \, J$
The frequency $\nu$ is calculated as $\nu = \frac{\Delta E}{h} = \frac{4.578 \times 10^{-19} \, J}{6.626 \times 10^{-34} \, J \, s} = 6.91 \times 10^{14} \, Hz$
The wavelength $\lambda$ is calculated as $\lambda = \frac{c}{\nu} = \frac{3.0 \times 10^{8} \, m \, s^{-1}}{6.91 \times 10^{14} \, Hz} = 4.34 \times 10^{-7} \, m = 434 \, nm$