Answer:
a) Â f = 809.5 Hz , Â b) Â Â f = 1619 Hz
Explanation:
The condition for destructive interference from the speakers is
       Δr / lam = Ф / 2π
       λ = Δr 2π / π
We substitute
      λ = 0.21 2 π/π
       Lam = 0.42 m
Now let's use the ratio of the speed of sound and the wavelength and the frequency
        v = λ f
        f = v /λ
        f = 340 / 0.42
        f = 809.5 Hz
b) in this case the intensity is maximum
     Lam = Δr 2π / 2π
     Lam = 0.21 m
     fF= v / lam
     f = 340 / 0.21
     f = 1619 Hz