You didn't mention the name of the variable C. But as you have provided the unit of Joule, I am considering it to be the energy. Now we know, in quantum mechanics, C=planck constant(h)xFrequency of the particle.

Now from theory Frequency= (Speed of light in vacuum/Wavelength of the particle). Therefore, C=h x (Speed of light in vacuum/Wavelength of the particle) which implies, Wavelength of particle= (h x speed of light in vacuum)/C. Now, we know h=6.626 x 10^-34 J-s & C=3.03*10^-19 & speed of light= 3x10^8 m/s, thus the wavelength of the particle will be- 656.03 x 10^-9 m = 656.03 nm.

I hope this helps. Wish you a great eve!