Two electric trains run at the same speed of

Two electric trains run at the same speed of $72 \mathrm{~km} \mathrm{~h}^{-1}$ along the same track and in the same direction with separation of $2.4$ $\mathrm{km}$ between them. The two trains simultaneously sound brief whistles. A person is situated at a perpendicular distance of $500 \mathrm{~m}$ from the track and is equidistant from the two trains at the instant of the whistling. If both the whistles were at $500 \mathrm{~Hz}$ and the speed of sound in air is $340 \mathrm{~m} \mathrm{~s}^{-1}$, find the frequencies heard by the person.