Answer:
a) The car was moving at a speed of 
b) The negative sign of
denotes that the observer is coming towards the police car which is the source of the sound.
c) 
Explanation:
Given:
- original frequency of the source,

- speed of the source,

- velocity of the obstacle car be,

- speed of sound,

- observed frequency,

<u>Using the equation from the Doppler's effect:</u>



a)
The car was moving at a speed of 
b)
The negative sign of
denotes that the observer is coming towards the police car which is the source of the sound.
c)
Now when, 
Then, 
Using the Doppler's eq.:


The value of the underlined 8 is, hundred billion's. Hope this helped!
Solution :
Acceleration due to gravity of the earth, g 

Acceleration due to gravity at 1000 km depths is :




= 8.23 m/s
Acceleration due to gravity at 2000 km depths is :




= 6.73 m/s
Acceleration due to gravity at 3000 km depths is :



= 5.18 m/s
Acceleration due to gravity at 4000 km depths is :




= 3.64 m/s
There is still air inside of a house, which is pushing the roof upwards, so the forces are equal and the roof is not crushed.