Two coherent violet wavelengths 400 nm long reach some points with a path difference of 1.2 µm.

Two coherent violet wavelengths 400 nm long reach some points with a path difference of 1.2 µm. What will happen at this point: strengthening or weakening of waves?

We will assume that if an integer number of wavelengths fit into the path difference, then we will get the maximum in the final version, and consequently the light will intensify.
But on the other hand, if the difference does not get an integer, then it will weaken, then we will get a minimum, which means the light will weaken.
Let’s count:
1.2 * 10-6 / 400 * 10-9 = 12/4 = 3.
Received an integer, a maximum occurs, the light is amplified.
Answer: light to intensify.