we know that
The measurement of the external angle is the semi-difference of the arcs it includes.
In this problem
[tex]21\°=\frac{1}{2}[arc\ RU-arc\ SU][/tex]
Solve for the measure of arc SU
[tex]42\°=[arc\ RU-arc\ SU][/tex]
[tex]arc\ SU=arc\ RU-42\°[/tex]
[tex]arc\ SU=119\°-42\°=77\°[/tex]
therefore
the answer is
The measure of the arc SU is [tex]77\°[/tex]
