Answer:
Hence lowest (nonzero) frequency that gives destructive interference in this case = 3400 Hz
Step-by-step explanation:
Since, the two are in out of phase,
their path difference is
d= nλ

Given d1= 2.75 m
D= 3.90 m


d_2= 2.76 m
2.76-2.75= 1×λ
λ= 0.01 m
0.01= 1*λ
λ =0.01
frequency ν = v/λ = 340/0.01
f= 3400 Hz
Hence lowest (nonzero) frequency that gives destructive interference in this case = 3400 Hz