If a diffraction grating produces its third-order bright band at an angle of 78.4 for light of

Question:

If a diffraction grating produces its third-order bright band at an angle of 78.4° for light of wavelength 681 nm, find
(a) The number of slits per centimeter for the grating and
(b) The angular location of the first-order and second-order bright bands.
(c) Will there be a fourth-order bright band? Explain.