Police radar guns measure the speed of moving vehicles by transmitting electromagnetic waves at a vehicle and detecting a Doppler shift in the reflected wave. Suppose a police radar transmits at a frequency of 23.0 GHz and receives a wave reflected from a car moving toward the radar at 72.0 mph. Find the frequency shift Δf = fₒ - fₛ (in Hz) between the observed (received) and source (transmitted) frequencies.