In this problem, we have that
the triangle URT is a right triangle
because
m by inscribed angle
arc UT=180 degrees ----> because the diameter TU divide the circle into two equal parts
so
mmIn the right triangle URT
we know
RU=15 -----> given
VT=13----> given ----> is the radius
TU=13(2)=26 ----> the diameter is two times the radius
solve for RT
Applying the Pythagorean theorem
TU^2=RT^2+RU^2
26^2=RT^2+15^2
RT^2=26^2-15^2
RT^2=451
RT=21.2 units