Answer: 100 degrees
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Explanation:
Refer to the figure below.
Focus on triangle ABF. Angle AFB = 40 is marked in red. Angle BAF = 90 is marked in blue. The missing angle is 180-90-40 = 50 which is angle ABF.
Inscribed angle ABF = 50 doubles to arc AC since this inscribed angle subtends the arc in question. I'm using the inscribed angle theorem.
arc AC = 2*(angle ABF) = 2*50 = 100 degrees
Note: We don't need the fact that angle AGD = 86
