What's the frequency of a blue light with this wavelength?

What is the frequency (in MHz) of a blue light whose wavelength is 4000 Angstroms?

You seem to be asking us to do your homework for you.

Hopefully this is too late to be any help with your assignment but might help you with your understanding.

An Angstrom is 1*10^-10 meters.

Light travels through vacuum at 300,000,000 meters/sec (and close enough to this speed in air to use this figure).

To convert wevelength to cycles/sec divide the speed of the wave in seconds by the wavelength.

3*10^8 / 4*10^-7 = 7.5*10^14

To get MHz divide by 10^6

7.5*10^8

or

750,000,000 MHz

More commonly this would be expressed in terahertz (10^12 cycles/sec) or THz

750 THz

Moreover, light with a wavelength of 4000 Angstroms would be violet not blue; this can probably be excused since many people can not discern blue from violet.