In the adjoining figure, $$PA, PB$$ are tangents. $$AR$$ is parallel to $$PB$$.

$$PQ = 6$$; $$QR = 18$$.
Length $$SB$$ = ------.

TO BE FILLED - figure required. It is not possible to determine reliably from the extracted figure which points are collinear with $$Q$$ (chord $$SR$$ versus line $$BR$$ through the tangent $$PA$$), so the similar-triangle relation coming from $$AR \parallel PB$$ cannot be set up with confidence.