Sodium light of wavelength 589 nm falls normally on a 2-cm-square diffraction grating ruled with 4000 lines

Question:

Sodium light of wavelength 589 nm falls normally on a 2-cm-square diffraction grating ruled with 4000 lines per centimeter. The Fraunhofer diffraction pattern is projected onto a screen at 1.5 m by a lens of focal length 1.5 m placed immediately in front of the grating. Find

(a) The positions of the first two intensity maxima on one side of the central maximum,

(b) The width of the central maximum, and