How do I find the spacing(d) of a grating in a diffraction grating experiment.

[mutineer123]

Homework Statement

http://www.xtremepapers.com/CIE/International%20A%20And%20AS%20Level/9702%20-%20Physics/9702_s02_qp_2.pdf

question no. 7b

Homework Equations

dsin∅=nλ

The Attempt at a Solution

I subtracted the two angles and got 26 degrees. So then I plugged it all into the equation:
dsin26=2 X 630x10^-9.
My answer is half the actual answer because the angle the answer sheet has is 13 degree. Why do they take it as 13 degree??

 

[ehild]

Look at the figure. The two angles correspond to the second-order maxima at both sides of the central maximum.
You need to know what angle is called ∅. ehild