Step-by-step explanation:
DeMoivre's theorem tells you the 7th roots of unity are the 7 points evenly spaced around (on) the unit circle, including 1∠0°. They will be located at angles ...
{0, 2π/7, 4π/7, 6π/7, 8π/7, 10π/7, 12π/7} . . . in radians
Given that the circle shown is graduated in increments of π/6, you will need to do some estimating as to the actual location of the root.
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The attached graph shows lines at increments of π/7 radians, so your graph will match every other one.