Frequency of any wave = (the wave's speed) divided by (its wavelength)
Speed of light = about (3 x 10 to the 8) meters per second, in vacuum.
Frequency = (3 x 10 to the 8) / (4.1 x 10 to the -12) = 7.317 x 10 to the 19 Hz.
That's the same as 73,170,000,000 GHz.
Note:
That's the answer for the information given in the question, but the information in
the question is incorrect by 5 orders of magnitude. The wavelength of violet light,
or anything in the visible range, is a few hundred 'nanometers' ... a few hundreds
of 10 to the -9 . So the number in the question should be 4.10 x 10 to the -7,
not -12, and the frequency should be 731,700 GHz.