Answer:
λ = V T S is distance between wavefronts, T is period of emitter
λ = 3.00E8 m/s * 1 / 2.40E10 = .0125 m
λ emitted by speed gun = .0125 m
But gun moves Vg * T between wavefronts
λ' = λ - Vg T where λ' is wavelength received by stationary observer
One can calculate T' from 3.00E8 m/s the period seen by observer, that is the period measured by the observer is the speed of light divided by the wavelength received by the observer
Now if the observer is moving the wavelength observed by the observer is λ'' = λ' - Vo T' since the observer will move a distance Vo T' between receiving wavefronts (T' has been calculated)
That is, the waves received by the tennis ball are shortened because of the motion of the tennis ball.
The speed of the wave measured is always 3.00E8 m/s