Answer: I believe the answer is 7.5 x 10^(14)Hz or 7.5E14Hz
Explanation: the formula for frequency is speed of the wave divided by the wavelength
f= c/wavelength
f- frequency
c-speed
(3 x 10^(8)) x (400 x 10^(-9))= 750,000,000,000,000 or 7.5E14 or 7.5 x10^(14)
