Suppose an oncoming ambulance moving at 105 km/h emits a steady 825 Hz sound from its siren

Suppose an oncoming ambulance moving at 105 km/h emits a steady 825 Hz sound from its siren.

(a) What frequency is received by a person watching the oncoming ambulance? The speed of sound on this day is 345 m/s.
(b) What frequency does she observe after the ambulance has passed?