I found the image that should have accompanied this problem.
The triangle inside the circle is an isosceles triangle. It has 2 equal sides and 2 equal angles.
Angle A = 70° , Angle C is also 70°. This leaves angle R having 40°.
The arc opposite angle R is twice the measure of its angle.
Angle R = 40°
Arc AC = 40° x 2 = 80°
The measure of arc AC is 80°.
