From Doppler effect we have that the frequency observed for the relation between the velocities is equivalent to the frequency observed. That is mathematically,
[tex]F_r = \frac{v}{v+v_s}F_s[/tex]
Here,
Speed of sound in water [tex]v = 1522m/s[/tex]
The Dolphin swims directly away from the observer with a velocity [tex]v_s = 8.5m/s[/tex]
Observed frequency of the clicks produced by the dolphin [tex]F_r = 2770Hz[/tex]
Replacing we have,
[tex]F_r = \frac{v}{v+v_s}F_s[/tex]
[tex]F_s = \frac{v+v_s}{v}[/tex]
[tex]F_s = 2770 (\frac{1522}{1522+8.5})[/tex]
[tex]F_s = 2754.61Hz[/tex]
Therefore the frequency emitted by the dolphin is 2754.61Hz