Grating Design your boss asks you to design a diffraction grating that will disperse the first-order visible

Question:

Grating Design your boss asks you to design a diffraction grating that will disperse the first-order visible spectrum through an angular range of 15.0° (see Example 36.4 in Section 36.5).
(a) What must the number of slits per centimeter be for this grating?
(b) At what angles will the first-order visible spectrum begin and end?