Answer:
TR = 3 units
Explanation:
PR = 12 units
PS = 9 units
PU = PS = 9 units (tangents drawn from an external point are congruent)
RU = PR - PU
RU = 12 - 9
RU = 3 units
TR = RU = 3 units (tangents drawn form an external point of a circle)
TR = 3 units