Chapter 7, Problem 2RQ

Suppose the inteference pattern shown in the figure below is produced by monochromatic light passing through a diffraction grating, that has 260 lines/mm, and onto a screen 1.40m away. What is the wavelength of light if the distance between the dashed lines is 180cm? nm

How many (whole) dark fringes will produced on on an infinitely large screen if red light (2)=700 nm) is incident on two slits that are 20.0 μm apart?

Can someone help me with this physics 2 problem thank you.