It is clear that the concave part forms the image at a distance $3 \lambda /2$ behind the lens, and the convex part forms an image at a distance $3 \lambda /2$ in front of it. Now, I do not understand what 'd' is supposed to be. If I assume it to be the distance at which image by convex lens is formed, I don't know what to do next. I can visualise a constructive interference happens there, but it happens only because of the rays from the convex part. So intensity should be 2I. But the answer they have given is 4I. I'll post their explanation below:
How do they correlate the distance between the images and the fact that maxima occurs at O? Thanks in advance.