1. The problem statement, all variables and given/known data I've attached the problem 2. Relevant equations d*sinθ=mλ 3. The attempt at a solution I've tried a couple of methods. I considered solving for m when θ=90 degrees => sinθ=1 because 90 degrees is the highest possible angle and will (in my mind at least) yield the highest order of spectrum by rounding the resultant value of m down to the nearest integer. But I don't know how to factor the two given wavelengths into all this. I then considered fiddling about with approximations like, at small angles, tanθ=sinθ=θ=y/L but to no avail. Can anyone help me on how to solve this? I don't even know how to picture the problem and draw a diagram.