In the question you are given the slit spacing of the diffraction grate. To find the fringe spacing of this grate you can use d=1/N where N is the slits per metre. In this question this would be 1/4.5x10^5 = d = 2x10^-6 m. To find the angle provided by the second order maximum, rearrange the equation nλ=dsinθ to make sinθ the subject.This would then become nλ/d = sinθ.To find sinθ, sub in the values given to you in the question; i.e. n=2, λ=650nm and d=2x10^-6 m.This gives sinθ=0.65.Use your calculator to find sin-1(0.65) to find θ=40.5°.