Doppler effect

A train passes by on a bridge blowing a whistle at 440 Hz. The position of the train whistle is described by the vector,

Here $t$ is the time measured in seconds. A girl ridding a bicycle has the position vector,

What frequency does the girl hear at time $t=$ s?

The speed of sound is $c=$ 340 m/s. Find the solution by plotting it graphically.