PR =12 and PQ =8. Therefore QR=12-8=4
And ST=18
Let SQ=x,then TQ=18-x
Here we have to use the chord intersection formula, which is
PQ*QR = SQ*TQ
8*4 = x(18-x)
Distributing x on 18-x
(x-16)(x-2)=0
x-16=0 , x-2=0
x=16,x=2
So SQ =16 and TQ=18-16=2
Therefore the answer is 2 .