Radial Velocity Question

Okay so the question is:
"If the radial velocity of a star is 5000km/s away from the observer, what is the shift in wavelength of a 656nm (rest λ) line in the hydrogen spectrum?"

FORMULA:
V= (Δλ / λ) C or you could just say it's V= ( Observed λ - Rest λ) C
V= the magnitude of radial velocity
C =speed of light
λ = rest wavelength
Δλ = difference of observed and rest wavelengths

so i get that the V = 5000km/s in this problem and C = 3 * 10^8 but I need an "observed wavelength" to subtract from the 656nm given in this problem. Also in the problem before this there WAS an observed wavelength of 660nm but if I were to plug that in then there would be no variable to solve. so what do i do???