The 7th bright fringe in the young's double slit experiment using a light of wavelength 550 nm shifts to the central maxima after covering the slits with two sh

The 7th bright fringe in the Young's double slit experiment using a light of wavelength 550 nm shifts to the central maxima after covering the slits with two sheets of different refractive indices n, and na but having same thickness 6 um. The value of In,-nal is (Round off to 2 decimal palces) 49.