You have to assume that the figure is a regular octagon, so has central angles equal to exterior angles equal to 45°.
6. A regular anything is circumscribed by a circle, so the measure of any inscribed angle is half the measure of the corresponding central angle. That is, ∠BET =∠TBA = (45°)/2, so their sum is 45°. ∠BOT is the supplement of that, 180° - 45° = 135°.
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7. ∠GIT + ∠ITS is the same measure as one internal angle of the octagon. That is the supplement of the exterior angle we identified above as being 45°.