The wavelength of a wave can be calculated using the formula:
wavelength = speed of light / frequency
where the speed of light is approximately 3.00 x 10^8 meters per second.
Substituting the given values, we get:
wavelength = (3.00 x 10^8 m/s) / (4.0 x 10^18 Hz)
wavelength = 7.5 x 10^-11 meters
Therefore, the wavelength of the X-ray with a frequency of 4.0 x 10^18 Hz is approximately 7.5 x 10^-11 meters