The relationship between wavelength [tex]\lambda[/tex], frequency f and speed of light c for an electromagnetic wave is
[tex]\lambda= \frac{c}{f} [/tex]
Using the data of the problem, we find
[tex]\lambda= \frac{3\cdot 10^8 m/s }{7.20 \cdot 10^{21} Hz}=4.17 \cdot 10^{-14} m [/tex]
