Suppose mica strip is on top slit and other one in bottom. So equate both path difference for central Maxima.
x=1250mm or 1.25m
As you are getting x large don't use approximation and use dsintheta in path difference. And tan theta=x/D
But answers given are
(a) 4.9×10^-4 m
(b) 0.021 cm on one side and 0.028 cm on other side
But b part doesn't matches