Explanation :
Frequency of gamma rays, [tex]\nu=7.5\times 10^{21}\ Hz[/tex]
The relation between the frequency and the wavelength is given by :
[tex]\lambda=\dfrac{c}{\nu}[/tex]
So, [tex]\lambda=\dfrac{3\times 10^8\ m/s}{7.5\times 10^{21}\ Hz}[/tex]
[tex]\lambda=4\times 10^{-14}\ m[/tex]
Hence, this is the wavelength of gamma rays.