1see on introducing the glass slab in between the 2 slits there is an optical path difference of ( u-1)*t, so both s1 and s2 will have a path difference of 0.12*t with respect to each other at THE CENTRE, now they will interfere constructively if 0.12*t = n* lambda solve to get 1248 nm = n *lambda so answer comes out to be 416 nm and 624 nm