So, you have all three angles of the triangle and you want to find the arc across from angle R. Since the\angles of a triangle total 180 degrees, each arc is twice the measure of the angle across from it on the other side. that being said, triangle ARC with an angle of 70 means its isosceles so it would have 2 angles at 70 degrees leaving an angle at 40 and since an arc is 2x an angle arc AC should be 80
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