The relationship between wavelength, frequency and speed of an electromagnetic wave is
[tex]\lambda= \frac{c}{f} [/tex]
where
[tex]\lambda[/tex] is the wavelength of the wave
c is the speed of light
f is the wave's frequency
The wave in this problem has frequency [tex]f=6.90 \cdot 10^{21} Hz[/tex], so its wavelength is
[tex]\lambda= \frac{c}{f}= \frac{3 \cdot 10^8 m/s}{6.90 \cdot 10^{21} Hz}=4.35 \cdot 10^{-14} m [/tex]
