Answer:
The distances of the three points from speaker A is  1). 1.713 m,  2). 4 m, 
3). 6.287 m.
Explanation:
Here we have 
Speed of sound, v = fλ
Where:
f = Frequency of sound and 
λ = Wavelength of the sound
Therefore, λ  = v/f =  = 4.573 m
 = 4.573 m
The two speakers are 8.0 m apart 
Let X be a point from speaker A on the line where we have constructive interference. Therefore,
(L - X) - X = n·λ
Which gives 
Therefore, we have, when n = 0, 

When n = 1 we have
 , which is the distance from speaker A, since from the nature of the calculation, if we selected X to be from speaker B, then there will be a point of constructive interference at 1.71 m from speaker B
, which is the distance from speaker A, since from the nature of the calculation, if we selected X to be from speaker B, then there will be a point of constructive interference at 1.71 m from speaker B
In other words since there is  a point of constructive interference at the mid point, we will have constructive interference at λ/2 on either side of the mid point
Therefore, the three points are;
4 - (4.573 m)/2, 4, 4+(4.573 m) 
The distances of the three points from speaker A is 
1). 1.713 m, 
2). 4 m, 
3). 6.287 m.