I really dont understand how youre supposed to do this question
Calculate the number of lines per metre on the grating.
Edit: nevermind i worked it out, you had to use your answer from the previous question divine by 2.5m, then rearrage d.sintheta = n.lambda for d which is d = (n.lambda/sintheta)
then take the reciprocal of your answer
But we haven't even met!