Answer:
The value is
![f_1 = 828 \ Hz](https://img.qammunity.org/2021/formulas/physics/college/9113hyl3zxr5knpjwwmx3a6nvehtn6ryo2.png)
Step-by-step explanation:
From the question we are told that
The speed is
The speed of the ambulance is
![u = 35 \ m/ s](https://img.qammunity.org/2021/formulas/physics/college/jn32fh5lsmrz7rpnqg6yo1ju16yxjl882r.png)
The frequency of the siren is
![f = 850 \ Hz](https://img.qammunity.org/2021/formulas/physics/college/lmd75m5gd4lp16eurcdg1bxiorco722x1b.png)
Generally from Doppler effect equation we have that
![f_1 = (v_s - v )/( v_s + u ) * f](https://img.qammunity.org/2021/formulas/physics/college/rdtoftughg2hkl83w96810gyc6ibtn2h4v.png)
Here
is the velocity of sound with the value
![v_s = 343 \ m/s](https://img.qammunity.org/2021/formulas/physics/college/heabqfbg4tnl88dy135ikadv1tu22op21i.png)
=>
![f_1 = (343 - 25 )/( 343 + 35 ) * 850](https://img.qammunity.org/2021/formulas/physics/college/5dlwx6p7ptej5gts3kq0jqk5xl1sz19dw2.png)
=>
![f_1 = 828 \ Hz](https://img.qammunity.org/2021/formulas/physics/college/9113hyl3zxr5knpjwwmx3a6nvehtn6ryo2.png)