Answer:
Since the beat frequency is 2/sec and one of the players is moving towards the stationary observer (he will hear 660 / sec from the trumpet player that is also stationary), the player moving towards the observer will produce a frequency of 662/sec as heard by the observer.
f' = f * v / (v - vs) where vs is the speed of the source
We can use 331 m/s as the speed of sound in dry air
f' v - f' vs = f v rearranging
vs = v (f' - f) / f'
vs = 331 m/s * (662 - 660) / 662 = 1 m/s
