According to question, light has wavelength, = 646 nm = 646 * 10-9 m.
distance of screen from slits, D = 8.75 m
Distance between central bright fringe and second order bright fringe is, y = 5.16 cm = 5.16 * 10-2 m. i.e for n = 2
Since, for maxima
Where d is distance between slits and n is order of maxima
Therefore, on rearranging
For minima, distance between central bright fringe and nth order minima (or dark fringe) is
putting all given values and calculated value(equation 1) in above equation we get
Therefore for n = 3
Therefore distance between the central bright fringe and third order minimum is 6.45 cm or 0.0645 m.