hm.sorry for late reply anyways..
take any general time "t" such dat angle rotated is

t
find the vel.. of point a ...which is WR
since the rod is rigid vel.. of a along the rod is same as the velocity of point B(attached to the block)
find the angle between the rod at b and the line joining the centre of the disc....
so vel of B=WR sin(90-Wt-Q)
since they hve given dat l >>> R
sinQ= rsinWt/l = tanQ = Q
so u get thew ans
phew!!!!!