Wavelength of H_a line emitted by hydrogen, λ = 6563 Aº = 6563 x 10⁻¹⁰ m.

Star’s red-shift, (λ ‘ —λ ) = 15 Aº = 15 x 10^-10 m

Speed of light, c = 3 x 10⁸ m/s

Let the velocity of the star receding away from the Earth be v.

The red shift is related with velocity as:

λ ‘ —λ = v λ /c

v = c/λ   x  (λ ‘ —λ )
= (3 x 10⁸ x 15 x 10^-10 )/(6563 x 10⁻¹⁰) =6.87 x 10⁵

Therefore, the speed with which the star is receding away from the Earth is 6.87 x 10⁵ m/s.