Circle theorems II — practice question

The circles centred at $O$ and $Q$ are shown touching at $P$, with $OPQ$ lying on one straight line. The straight line $ORT$ cuts the smaller circle at $R$ and is a tangent to the larger circle at $T$. $OR = 4\,\text{cm}$ and $RT = 6\,\text{cm}$. The radius of the larger circle is $x\,\text{cm}$.