1) Destructive interference
The condition for constructive interference to occur is:
 (1)
 (1)
where
 is the path difference, with
 is the path difference, with
 is the distance of the point from the first source
 is the distance of the point from the first source
 is the distance of the point from the second source
 is the distance of the point from the second source
m is an integer number
 is the wavelength
 is the wavelength
In this problem, we have

So let's use eq.(1) to see if the resulting m is an integer

It is not an integer so constructive interference does not occur.
Let's now analyze the condition for destructive interference:
 (2)
 (2)
If we apply the same procedure to eq.(2), we find

which is an integer: so, this point is a point of destructive interference.
2) Constructive interference
In this case we have

So the path difference is

Using the condition for constructive interference:

Which is an integer, so this is a point of constructive interference.
3) Destructive interference
In this case we have

So the path difference is

Using the condition for constructive interference:

This is not an integer, so this is not a point of constructive interference.
So let's use now the condition for destructive interference:

which is an integer: so, this point is a point of destructive interference.