Let PQ and PR touch the circle at points S and U respectively. Join O with P, Q, R, S and U
We have OS = OT = OU = 6cm
QT = 12 cm and TR = 9cm
QR = QT +TR = 12cm + 9cm = 21 cm
Now QT = QS = 12 cm (tangent from the same point)
TR = RU = 9cm
Let PS = PU = x cm
Then PQ=PS+SQ- (12 + x)cm and PR = PU+RU= (9+x)cm
It is clear that
Thus PQ = (12 + 10.5) cm and PR = (9 + 10.5) cm = 19.5 cm