The report stated that majority of the candidates could not answer part (a) of this question correctly which showed a poor knowledge of circle theorems.  They were expected to join X and Y to O.  This made <XOY and <XAY opposite angles of a cyclic quadrilateral and are supplementary i.e. a° + <XOY = 180°.  Also, <XOY = 2b° i.e. angle at the centre of a circle is twice angle at the circumference.  This implied that 2b° = 180 - a° or a° + 2b°  = 180°. 
It was reported that candidates performed better in part (b) than in part (a).  A good number of them were able to find the tangent of the angle at R and hence the angle at R as 36.87°.  Thus, they were able to obtain /TR/ = 8 cos R i.e. 8 cos 36.87° = 6.4cm.