The relationship between the frequency and wavelength of and electromagnetic wave is c(speed of light) = f(frequency) * lambda(wavelength). c = 3 x 10^8 m/s and, in this case, the wavelength is .84 nm or 8.4 x 10^-10. Plugging in to the equation above we see the frequency of the wave is 3.57 x 10^17 s^-1 or Hz.