Answer:
f λ = c = 3.00E8 frequency * wavelength = speed of light
λ = c / f = 3.00E8 / 4.0E14 = 7.5E-7 m
Note 1 angstrom = 1.00E-10 m and 1 milli-micron = 1.00E-9 m
So the wavelength would be 7500 angstroms
The visible spectrum is 4000 A - 7000 A or 400 - 700 milli-microns