A beam of light is incident normally on a diffraction grating of width 5 cm. it is found that at 30°, the order diffraction maximum for 7000 a is super-imposed

A beam of light is incident normally on a diffraction grating of width 5 cm. It is found that at 30°, the order diffraction maximum for 7000 A is super-imposed on the (n+1" order for 2= 6000 A. The numbers of lines per cm does the grating have? (a) 15000o b) 6164 (c) 10417 (d) 12328