Answer:
frequency 1 = 334.30 Hz
frequency 2 = 1002.92 Hz
Explanation:
Given data 
speaker distance y = 0.513 m
microphone distance D = 1.80 m
to find out
lowest two frequencies 
solution
we know velocity of sound is 343 m/s
so we consider point x 
so at 1st speaker distance from x   = D + (y/2)
1st speaker distance from x   = 1.80 + (0.513/2) = 2.0565 m   .....1
and
 at 2nd speaker distance from x   = D - (y/2)
2nd speaker distance from x   = 1.80 - (0.513/2) = 1.5435 m     .........2
so destructive interference from 1 and 2  we know
1st - 2nd = ( m + 0.5 ) wavelength
 2.0565 m - 1.5435 m = (  0+ 0.5) wavelength 
wavelength  = 1.026 m
so here 1st min frequency will be 
frequency 1 = velocity of sound / wavelength
frequency 1 = 343 / 1.026 =334.30 Hz
and 
2nd  min frequency will be
frequency 2 = 
 2.0565 m - 1.5435 m = (  1 + 0.5) wavelength 
wavelength  = 0.342 m
frequency 2 =  velocity of sound / wavelength
frequency 2 = 343 / 0.342 = 1002.92 Hz