The measure of an angle formed by two secants, two tangents, or a secant and a tangent drawn from a point outside a circle is equal to half the difference of the measures of the intercepted arcs.
This theorem states that 27 would equal to half the difference between 128 and the measure of arc AC.
27 = 1/2 (128 - AC)
54 = 128 - AC
AC = 74
