Are you allowed to use the fact that the internal angles of a quadrilateral add up to 360 degrees?
If so, note that RSXT is a quadrilateral whose angles you know all of except for the one you are being asked to find.
If measurement of arc ST = 71 degrees, calculate m<SXT?
a.) 54.5 degrees
b.) 71 degrees
c.) 109 degrees
d.) 142 degrees