A car is parked 21.0 m directly south of a railroad crossing. A train is approaching the crossing from the west, heading directly east at a speed of 50.0 m/s. The train sounds a short blast of its 309-Hz horn when it reaches a point 18.0 m west of the crossing. What frequency does the car's driver hear when the horn blast reaches the car? The speed of sound in air is 343 m/s. (Hint: Assume that only the component of the train's velocity that is directed towards the car affects the frequency heard by the driver.)