With what speed should a galaxy move with respect to us to that the sodium line at `589.0 nm` is observed at `589.6 nm` ?

Explanation

Here, `v = ?, lambda = 589.0 nm`
`Delta lambda = 589.6 - 589.0 = 0.6 nm`
From `v = (Delta lambda)/(lambda) xx c = (0.6)/(589.0) xx 3 xx 10^(8) m//s`.
`v = 3.06 xx 10^(5) m//s = 306 km//s`
The galaxy is moving away from us, as apparent `lambda` increases.