wavelength of laser light?

Question

You have a laser with wavelength (λ) 640 nm. You shine it through a diffraction grating of 400 lines per millimeter. You measure the distance to a screen to be 20.3 cm. You measure the second dot in millimeters from the center dot.
What is the wavelength of the laser light?
__________________[nanometer].
Use three significant figure or N/A if not enough information is given.

Answer 1

Trick question.

The output wavelength from the diffraction will be the same as the input.

But to find out more about diffractions, you can check out:
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/gratcal.html