Try a second example:

Suppose a galaxy (or any other object) is moving away with a speed of 100 km/s. To what wavelength is the H alpha line shifted to?
[assume the rest wavelength of H alpha is 656.25 nm]

Doppler's Law: (lambda - lambda_rest) / lambda_rest = v / c

where lambda_rest = 656.25 nm ,
v = 100 km/s , and
c = 3 x 10⁵ km/s

So
(lambda - 656.25 nm / 656.25 nm ) = ( 100 km/s ) / (3 x 10⁵ km/s)
(lambda - 656.25 nm / 656.25 nm ) = (3.333 x 10^-4)
(lambda - 656.25 nm) = 656.25 nm x (3.333 x 10^-4)
(lambda - 656.25 nm) = 0.21875 nm
lambda = 656.25 nm + 0.21875 nm
lambda = 656.46875 nm , or
lambda = 656.47 nm

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Ok, for those of you who want a challenge:

Why should the redshift really be called a redstretch?