You’ve found an unlabeled diffraction grating. Before you can use it, you need to know how many lines per mm it has. To find out, you illuminate the grating with light of several different wavelengths and then measure the distance between the two first-order bright fringes on a viewing screen 150 cm behind the grating. Your data are as follows:

Wavelength (nm)....................Distance (cm)
430...............................................109.6
480...............................................125.4
530...............................................139.8
580...............................................157.2
630...............................................174.4
680...............................................194.8

Use the best-fit line of an appropriate graph to determine the number of lines per mm.