What speed should a galaxy move with respect to use so that the sodium line at 589.0 nm is oberved at 589.6 nm ?

Explanation

Since `vlambda= c,(Deltav)/(v) =-(Deltalambda)/lambda`
(for small changes in v and `lambda`)
For `Deltalambda =589.6 -589.0 = +0.6 nm`
`(Deltav)/(v) = - (Deltalambda)/lambda = - v/c` or
`v = +c (0.6/589.0) = +3.06 xx 10^(5) ms^(-1)`
Therefore. the galaxy should move away from us with speed 306 km/s.