The applicable expression is:
Sin Ф = Wavelength/Grating spacing
Where,
Ф = Angle of red light
Wavelength = 640 nm = 640*10^-9 m
Grating spacing = 1 cm/2010 = 0.01/2010 = 4.975 *10 ^-6 grooves per meter
Therefore,
Ф = Sin ^-1 [(640*10^-9)/(4.975*10^-6)] = 7.39°