The answer is 3.9E-33 kg, I'm looking for a solution to this problem. Any help (formulas, step-by-step solutions) would be great!
If it's traveling at the speed of light, its rest mass is zero, such as a photon. It could also be a gluon, graviton, or approximately it could be a neutrino. But if you're looking for an answer that matches that number, it's the m in
E = mc^2
Use the formula that relates de Broglie wavelength to energy, and you can find the energy. Then you can find the "mass".
E^2 = (p^2 x c^2) + (m^2 x c^4)
where p is the momentum and equal to (h x omega)/2 * PI
h is plancks constant
as far as I remember