Monochromatic light from a point source illuminates two parallel, narrow slits. The centres of the slip openings are 0.8mm apart. An interference pattern forms on a screen placed parallel to the plane of the slits and 49 cm away. The distance between two adjacent dark interference fringes is 0.30 mm.
a) Calculate the wavelength of the light.

I got 490 nanometers, the book got 540. Whats the correct answer?

I used the formula X/L = lambda/d

Thanks