When person is observing destructive interference at 0.20 m distance from the equidistant position then we can say that path difference must be equal to half of the wavelength
now we will have
[tex]\frac{\lambda}{2} = \frac{yd}{L}[/tex]
now we know that
y = 0.20 m
d = 2.4 m
L = 10 m
now here we have
[tex]\frac{\lambda}{2} = \frac{0.20\times 2.4}{10}[/tex]
[tex]\lambda = 0.096 m[/tex]
now frequency of wave is given as
[tex]f = \frac{v}{\lambda}[/tex]
[tex]f = \frac{343}{0.096} = 3573 Hz[/tex]