When yellow sodium light, = 589 nm, falls on a diffraction grating, its first-order peak on

When yellow sodium light, λ = 589 nm, falls on a diffraction grating, its first-order peak on a screen 72.0 cm away falls 3.32 cm from the central peak. Another source produces a line 3.71 cm from the central peak. What is its wavelength? How many slits / cm are on the grating?