To answer this question, we can use the wave equation formula:
http://www.antonine-education.co.uk/Pages/Physics_1/Particles/PP03/particles_page_3.htm
Where lamda is the wavelength and it has units of meters. c is the speed of light and it has a value of 3.0xx10^8 m/s. f represents the frequency and it has units of Hertz or reciprocal seconds (1/s).
We know the frequency, f and the speed of light since that is a constant. All we have to do is plug in the known values and solve for lambda:
lambda = (3.0xx10^8 m/cancel"s")/((6.21xx1018)/cancel"s")
lambda = (3.0xx10^8 m/cancel"s")/(6321.78/cancel"s")
lambda = 4.75xx10^4 m