Here, λ = 6563 Å, Δλ = 15 Å, c = 3 × 108ms–1
Since the star is receding away, hence its velocity v is negative (i.e. if Δλ is positive, v is negative)
∴ Δλ = −

vλ

c

or v = −

cΔλ

λ

= -

3×108×15

6563

= - 6.86 × 105ms−1
Here, negative sign shows recession of star.