4th order dark fringe and 2nd order bright fringe spaced 2.5 cm aparte appear on the screen placed 1 m behind the double slit. What is the spacing of the slits if the wavelegth ofincident light is 500nm?

First I converted the numbers:
x= .025m
wavelength= 5x10^-7m
L=1m
d=?

I used d= wavelenght*L/x and got 2x10^-5 m
but I got this problem wrong and I was wondering how you would correctly solve this problem. Did I assign the correct numbers with the correct letters?