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Just a quick reminder, the site is for studying, not for tests or quizzes, but I assume this one is just a quick study test,

the line AC is tangent to the circle, and is touching the radius AB at A.

now, the point of tangency, namely where a tangent and radius chord meet, is always a right-angle, namely the ∡A is 90°, and we also know that ∡C is 71°, well, since the sum of all interior angles in a triangle add up to 180°, ∡B = 180 - ∡A - ∡C.