Chem calculate the wavelength of an electromagnetic wave with a frequency of (in pic)

we have,

wavelenght=c/f

where c= 3×10^8 m/s

f=6.3×10^12 s^-1

so wavelength=(3×10^8)/(6.3×10^12)

=0.476×10^-4 m

if you mean hydrogen and oxygen then water is the product