The measure of an angle formed by a secant and a tangent drawn from a point outside the circle is half the the difference of the intercepted arcs.
[tex]21^o= \frac{arc \ RU-arc \ SU}{2}= \frac{119^o-arc \ SU}{2} \ \to \ \\ \\ 42= 119-arcSU \\ arc \ SU=119-42=77^o[/tex]
