Answer:
v = 8.8 m /s
Step-by-step explanation:
For listener and source going away from each other the formula of Doppler effect is as follows

V is velocity of sound , v is velocity of listner and source of sound
f₀ is apparent frequency and f is real frequency
V = 343 , v = ? ,f = 210 , f₀ = 221
Put these value in the relation above
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v = 8.8 m /s