Australian AMC 2009 - Senior Question 12

A rectangle $PQRS$ has $PQ=2x$ cm and $PS=x$ cm. The diagonals $PR$ and $QS$ meet at $T$. $X$ lies on $RS$ so that $QX$ divides the pentagon $PQRST$ into two sections of equal area. The length, in centimetres, of $RX$ is