So I've been trying to input the correct answer for quite some time but I've kept getting it wrong. The answer I believe it has to be is 0.4 mm. In my assignment, it asks for the final answer to be in millimeters.

26. | Parallel light rays with a wavelength of 600 nm fall on a single slit. On a screen 3.00 m away, the distance between the first dark fringes on either side of the central maximum is 4.50 mm. What is the width of the slit