Calculate the line spacing of the diffraction grating (d) Diffraction grating are usually defined in "lines per millimetre" Convert this to lines per metre (multiply by 1000) then take the reciprocal to get d. For a monochromatic light source the first order image will be a line, assuming the light has been passed through a slit. Measure the angle between the zero order line and the first order line, this gives angle theta.
The wavelength= d multiplied by the sine of angle theta.