Gamma-ray bursters emit pulses of gamma rays with high energies. The frequency of the most energetic pulses has been measured at around 3.0 x 1021 Hz. What is the wavelength of these gamma rays?

Speed = wavelength x frequency

Gamma rays are a part of the electromagnetic spectrum hence travel at the speed of light in vacuum.

So,

3 x 10^8 m/s = wavelength x 3.0 x 10^21 Hz

Rearranging,

wavelength = 3 x 10^8 m/s / 3.0 x 10^21 Hz = 1 x 10^-13 m