The police car with its 300- siren is moving away from the warehouse at a speed of 20.0?

you are missing all of the units, so the question is meaningless.

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You have to do the calculation twice, since the sound is reflected:

1st:
Source: Police Car
Listener: warehouse

2nd:
Listener: warehouse
Source: Police Car

Beware of the signs (+/-) of velocity, it changes across the cases since the perspective is different.

Velocity of sound = 344 m/sec. Wavelength = 344/300 = 1.147metres.
20???? Metres/ sec??? kph???? I'll try kph.
20kph = 20,000/3600 = 5.55 m/sec.
Total velocity = 349.55 m'sec.
349.55/344 = 1.0129.
1.147 x 1.0129 = 1.162m.
Frequency with wavelength 1.162m = 344/1.162 = 296.04Hz.

Employ the Doppler eqn: fl = fs[(c±vl)/(c-/+vs)

where,

frequency heard by listener: fl = ?
frequency of source: fs = 300 Hz
velocity of listener: vl = 20 m/s
velocity of source: vs = 0
velocity of sound in air: c = 344 m/s

The source will be the origin of the reflected signal (i.e.the
warehouse). So that, velocity of source: vs = 0

The listener is the policeman moving away from the source
with velocity: vl = 20 m/s

Therefore, the Doppler eqn. becomes:

fl = fs [(c – vl)/c] = 300[(344-20)/344] = 282.56 Hz

where, fl is the frequency of the reflected sound heard by
the police officer.