4.00 m/s
First, calculate the over all speed of the bird in m/s by dividing the distance by the time. Since I want m/s, I'll first convert the distance to meters and the time to seconds.
6000 m / 1200 s = 5 m/s
Now the ground speed of the bird is it's flight speed minus the wind velocity. So make an equation showing this and the ground speed.
5 m/s = 9.00 m/s - x
And solve for x
5 m/s = 9.00 m/s - x
Add x to both sides
x + 5 m/s = 9.00 m/s
Subtract 5 m/s from both sides
x = 4.00 m/s
Therefore the wind velocity is 4.00 m/s